Thanks to Christof Schulze for noticing the problem.

When adding unreachable routes and setting the RTNH_F_ONLINK flag, a
device is required to be specified. In Linux kernel 4.16 support for
this flag was added. Until now it was ignored.
If RTNH_F_ONLINK is specified while the device is missing, newer kernels
will respond with No such device.
The result is:
* spam in the log file
* missing routes for both ipv4 and ipv6

The check for a TLV going beyond the end of the packet was off by two.
A malformed packet could possibly cause babeld to read two octets beyond
the end of the read buffer.

While technically a buffer overflow, this is most probably not
exploitable, since it is a read-only overflow.  At worst, it would
cause two octets of garbage to be parsed and treated as valid data.

This fixes a bug that was introduced in commit 4f4e3cba, and prevented
non-source-specific IPv4 routes from being redistributed.  Thanks
to Niklas Yann Wettengel for the detective work.

Point-to-point interfaces are bound to two link-local addresses: the
local address and the peer address [1].  The former is advertised with
an IFA_LOCAL TLV and the latter with an IFA_ADDRESS TLV.

[1] $ip addr show [...]
    inet6 fe80::1234 peer fe80::5678/128 scope link

Since send_request was buggy, these weren't doing anything.  Don't change
the behaviour, sending wildcard requests at startup is not a good idea.

When we lose a route and have no feasible alternate, we should send requests
straight away rather than waiting for a periodic update.  If we have an
unfeasible route, send according to that route and resend, otherwise do
a multicast.

Reported by janus.

This also removes special casing of late Hellos.

- Hello is not ignored if there is a mandatory sub-TLV,
- non-wildcard Updates also,
- Duplicated check for Requests,
- wrong size for the beginning of sub-TLVs for Seqno Requests,
- wrong size for source specific Requests and Seqno Requests.

That patch was fixing a little bug, but makes another bigger one.
The previous patch fixes both.

This reverts commit e687a58f.