better error checking for LOCAL_PEERCRED

better error checking for LOCAL_PEERCRED

This patch improves the error checking in the LOCAL_PEERCRED version
of virNetSocketGetUNIXIdentity, used by FreeBSD and Mac OSX.

1. The error return paths now correctly unlock the socket. This is
implemented in exactly the same way as the SO_PEERCRED version,
using "goto cleanup"

2. cr.cr_ngroups is initialised to -1, and cr.cr_ngroups is checked
for negative and overlarge values.

This means that if the getsockopt() call returns success but doesn't
actually update the xucred structure, this is now caught. This
happened previously when getsockopt was called with SOL_SOCKET
instead of SOL_LOCAL, prior to commit 5a468b3, and resulted in
random uids being accepted.

Signed-off-by: Eric Blake <eblake@redhat.com>