virnetsocket: fix getsockopt on FreeBSD

virnetsocket: fix getsockopt on FreeBSD

aa0f099 introduced a strict error checking for getsockopt and it
revealed that getting a peer credential of a socket on FreeBSD
didn't work. Libvirtd hits the error:
  error : virNetSocketGetUNIXIdentity:1198 : Failed to get valid
  client socket identity groups

SOL_SOCKET (0xffff) was used as a level of getsockopt for
LOCAL_PEERCRED, however, it was wrong. 0 is correct as well as
Mac OS X.

So for LOCAL_PEERCRED our options are SOL_LOCAL (if defined) or
0 on Mac OS X and FreeBSD. According to the fact, the patch
simplifies the code by removing ifdef __APPLE__.

I tested the patch on FreeBSD 8.4, 9.2 and 10.0-BETA1.

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>