Add to neotest bench-net command that performs latency measurments at
ping and TCP levels. Example output:

	x/src/lab.nexedi.com/kirr/neo/go/neo/t$ ./neotest bench-net neotest@rio:9
	node:
	cluster:        deco-rio

	*** link latency:

	# deco ⇄ rio (ping 16B)
	PING rio (192.168.0.8) 16(44) bytes of data.

	--- rio ping statistics ---
	25705 packets transmitted, 25705 received, 0% packet loss, time 2999ms
	rtt min/avg/max/mdev = 0.080/0.097/0.220/0.011 ms, ipg/ewma 0.116/0.095 ms
	Benchmarkpingrtt-/16B-min 1 0.080 ms/op
	Benchmarkpingrtt-/16B-avg 1 0.097 ms/op
	# POLL·3 C1·476 C1E·60917 C3·53 C6·132 C7s·0 C8·203 C9·0 C10·141

	...

	*** TCP latency:

	# deco ⇄ rio (lat_tcp.c 1B  -> lat_tcp.c -s)
	Benchmarktcprtt(c_c)-/1B 1 116.1743 µs/op       # TCP latency using rio: 116.1743 microseconds  # POLL·6 C1·892 C1E·65748 C3·80 C6·165 C7s·0 C8·339 C9·0 C10·444
	Benchmarktcprtt(c_c)-/1B 1 117.2896 µs/op       # TCP latency using rio: 117.2896 microseconds  # POLL·4 C1·1063 C1E·67647 C3·64 C6·77 C7s·0 C8·144 C9·0 C10·209
	Benchmarktcprtt(c_c)-/1B 1 117.5331 µs/op       # TCP latency using rio: 117.5331 microseconds  # POLL·1 C1·954 C1E·76866 C3·96 C6·88 C7s·0 C8·206 C9·0 C10·246
	Benchmarktcprtt(c_c)-/1B 1 117.6509 µs/op       # TCP latency using rio: 117.6509 microseconds  # POLL·4 C1·731 C1E·84210 C3·103 C6·93 C7s·0 C8·180 C9·0 C10·187
	Benchmarktcprtt(c_c)-/1B 1 116.8125 µs/op       # TCP latency using rio: 116.8125 microseconds  # POLL·9 C1·550 C1E·79544 C3·110 C6·213 C7s·0 C8·508 C9·0 C10·475

	...

And its summary via benchstat:

	name                 time/op
	pingrtt-/16B-min     80.0µs ± 0%
	pingrtt-/16B-avg     97.0µs ± 0%
	-pingrtt/16B-min     79.0µs ± 0%
	-pingrtt/16B-avg      112µs ± 0%
	pingrtt-/1452B-min    241µs ± 0%
	pingrtt-/1452B-avg    303µs ± 0%
	-pingrtt/1452B-min    266µs ± 0%
	-pingrtt/1452B-avg    303µs ± 0%
	tcprtt(c_c)-/1B       117µs ± 1%
	tcprtt(c_go)-/1B      122µs ± 2%
	-tcprtt(c_c)/1B       117µs ± 1%
	-tcprtt(c_go)/1B      121µs ± 5%
	tcprtt(c_c)-/1400B    392µs ± 4%
	tcprtt(c_go)-/1400B   363µs ±18%
	-tcprtt(c_c)/1400B    412µs ±21%
	-tcprtt(c_go)/1400B   391µs ±38%
	tcprtt(c_c)-/1500B    271µs ±18%
	tcprtt(c_go)-/1500B   290µs ±21%
	-tcprtt(c_c)/1500B    282µs ±16%
	-tcprtt(c_go)/1500B   334µs ±24%
	tcprtt(c_c)-/4096B    711µs ± 5%
	tcprtt(c_go)-/4096B   737µs ± 5%
	-tcprtt(c_c)/4096B    740µs ± 2%
	-tcprtt(c_go)/4096B   711µs ± 7%

Latencies here are not good because for this run on rio interrupt mitigation
was not tuned (see below). By the way, analyzing ping RTT latencies on our
shuttle machines (similar to rio) resulted in the following kernel patch

	https://git.kernel.org/linus/509708310c (released with Linux 4.15)

to fix/being able to adjust interrupt mitigation on Realtek NICs.

While at networking topic, teach info/info-local to show related
information about node's NICs. Example lines output for deco:

	nic/eth0: Intel Corporation Ethernet Connection I219-LM rev 21
	nic/eth0/features: rx tx sg tso !ufo gso gro !lro rxvlan txvlan !ntuple rxhash ...
	nic/eth0/coalesce: rxc: 3μs/0f/0μs-irq/0f-irq,  txc: 0μs/0f/0μs-irq/0f-irq
	nic/eth0/status:   up, speed=1000, mtu=1500, txqlen=1000, gro_flush_timeout=0.000µs
	nic/wlan0: Intel Corporation Wireless 8260 rev 3a
	nic/wlan0/features: !rx !tx sg !tso !ufo gso gro !lro !rxvlan !txvlan !ntuple !rxhash ...
	nic/wlan0/coalesce: rxc: ?,  txc: ?
	nic/wlan0/status:   down, speed=?, mtu=1500, txqlen=1000, gro_flush_timeout=0.000µs
	WARNING: nic/wlan0: TSO not enabled - TCP latency with packets > MSS will be poor

for rio:

	nic/eth0: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller rev 06
	nic/eth0/features: rx !tx !sg !tso !ufo !gso gro !lro rxvlan txvlan !ntuple !rxhash ...
	nic/eth0/coalesce: rxc: 200μs/4f/0μs-irq/0f-irq,  txc: 200μs/4f/0μs-irq/0f-irq
	nic/eth0/status:   up, speed=1000, mtu=1500, txqlen=1000, gro_flush_timeout=0.000µs
	WARNING: nic/eth0: TSO not enabled - TCP latency with packets > MSS will be poor
	WARNING: nic/eth0: RX coalesce latency is max 200μs - that will add to networked request-reply latency
	nic/eth1: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller rev 06
	nic/eth1/features: rx !tx !sg !tso !ufo !gso gro !lro rxvlan txvlan !ntuple !rxhash ...
	nic/eth1/coalesce: rxc: 0μs/1f/0μs-irq/0f-irq,  txc: 0μs/1f/0μs-irq/0f-irq
	nic/eth1/status:   down, speed=?, mtu=1500, txqlen=1000, gro_flush_timeout=0.000µs
	WARNING: nic/eth1: TSO not enabled - TCP latency with packets > MSS will be poor

The warning about "RX coalesce latency is max 200μs ..." says that on
receive path eth0 will be coalescing incoming frames for up to 200μs and
this way this delay will be added to overal latency. (for small frames
Realtek NICs do not coalesce interrupts - see details in the kernel patch).

Networked performance (raw and NEO) was not discussed in
http://navytux.spb.ru/~kirr/neo.html at all, but for the reference the
importance of C-states for performance was first found via this
networking latency benchmarks. Links on C-states topic:

	http://navytux.spb.ru/~kirr/neo.html#cpu-idle-c-states
	http://navytux.spb.ru/~kirr/neo.html#appendix-ii-cpu-c-states

Some draft history related to this patch:

	lab.nexedi.com/kirr/neo/commit/e8e395ae	X neotest: Move network benchmarking into separate function + add `neotest bench-net`
	lab.nexedi.com/kirr/neo/commit/a971231c	X neotest/info: Handle USB NICs
	lab.nexedi.com/kirr/neo/commit/5dd3d1ab	X neotest: sort NIC names
	lab.nexedi.com/kirr/neo/commit/9888f047	X neotest: Do not crash if kernel is too old to support gro_flush_timeout
	lab.nexedi.com/kirr/neo/commit/3a1bdf4a	X bench-remote / tcp : std benchmark output
	lab.nexedi.com/kirr/neo/commit/9450b6db	X bench-remote / ping += std bench output
	lab.nexedi.com/kirr/neo/commit/68d5b015	X show gro_flush_timeout + friends
	lab.nexedi.com/kirr/neo/commit/4c815af9	X neotest: Show NIC features and emit warning if !TSO
	lab.nexedi.com/kirr/neo/commit/659ce938	X neotest: Adjust ping and TCP RR sizes to fit 1 Ethernet frame, etc...
	lab.nexedi.com/kirr/neo/commit/ded384cb	X neotest += `lat_tcp.go -s`
	lab.nexedi.com/kirr/neo/commit/59d46504	X neotest += lat_tcp
	lab.nexedi.com/kirr/neo/commit/67fc3440	X show small (56B) and full-packet (1472B) ping link latencies