Move the common Getsockopt* implementations to syscall_unix.go instead
of duplicating them in syscall_bsd.go and syscall_linux.go. This also
makes all of them available on Solaris and adds GetsockoptByte on Linux.

Change-Id: Ibf49b27be46be579e63e3b3e2ebc1504e5228d5f
Reviewed-on: https://go-review.googlesource.com/104695
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>