ipc: set gid on unix sockets

When creating a unix socket it's default gid is that of the parent
directory. If the SOCKETDIR is owned by root:wheel with 1777 mode
some of the pacemaker daemons end up unable to communicate with one
another due to having insufficient permissions on the sockets.

This can be fixed by setting the client sockets gid to the primary
group of the server socket owner it's attempting to connect to. And,
on the server side by setting the gid to the already captured gid
stored in the connection info. This ensures that regardless of who
owns the socket directory, as long as the applications have r/w
access to it they should work.