rpc: fix getsockopt for LOCAL_PEERCRED on Mac OS X

rpc: fix getsockopt for LOCAL_PEERCRED on Mac OS X

This fixes the following error:
  error : virGetUserEnt:703 : Failed to find user record for uid '32654'

'32654' (it's random and varies) comes from getsockopt with
LOCAL_PEERCRED option. getsockopt returns w/o error but seems
to not set any value to the buffer for uid.

For Mac OS X, LOCAL_PEERCRED has to be used with SOL_LOCAL level.
With SOL_LOCAL, getsockopt returns a correct uid.

Note that SOL_LOCAL can be found in
/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/un.h.

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>