Converting a syscall.Errno to an interface is
a significant source of allocations in os/exec.

Elsewhere in the tree, we have pre-allocated errors
for common errno values. Use the same trick here.

This CL makes yet another copy of this code.
The problem is that there isn't really a great place to share it.

The existing copies are in:

cmd/vendor/golang.org/x/sys/unix
cmd/vendor/golang.org/x/sys/windows
cmd/vendor/golang.org/x/sys/windows/registry
internal/syscall/windows
internal/syscall/windows/registry
syscall

internal/poll can't import from cmd/vendor, and cmd/vendor
can't import from internal/*, so we can ignore cmd/vendor.

We could put the unix version in internal/syscall/unix
and then have a platform-independent wrapper in internal/syscall.
But syscall couldn't use it; internal/syscall/* depends on syscall.
So that only allows code re-use with internal/syscall/windows/*.

We could create a new very low level internal package, internal/errno.
But syscall couldn't use it, because it has to import syscall
to get access to syscall.Errno.
So that only allows code re-use with internal/syscall/windows/*.

It's not clear that that any of these options pulls its weight.

The obvious and "correct" place for this is syscall.
But we can't export syscall's version, because package syscall is frozen.

So just copy the code. There's not much of it.

name            old alloc/op   new alloc/op   delta
ExecHostname-8    6.15kB ± 0%    6.13kB ± 0%  -0.38%  (p=0.000 n=20+19)

name            old allocs/op  new allocs/op  delta
ExecHostname-8      34.0 ± 0%      31.0 ± 0%  -8.82%  (p=0.000 n=20+20)

Fixes #30535

Change-Id: Idd31c7cced6e15387acc698ffc011e1b7b479903
Reviewed-on: https://go-review.googlesource.com/c/164971
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>