syscall: allow abstract unix socket to use the full Path len

syscall: allow abstract unix socket to use the full Path len

The previous implementation forced all Unix socket to have a name
strictly shorter than len(sa.raw.Path) to allow a terminating NULL
byte to be added. This requirement does not apply to abstract socket
names under Linux, so for this case we allow the full length.

Fixes #21965

Change-Id: I1d1f58b6b6172d589428c7230cfeae984de78b4b
Reviewed-on: https://go-review.googlesource.com/66190
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>