Allow group users write to the controlling UNIX-socket.

Hello All!
I'd like to re-propose one of our old patches for merging. This patch allows users in group "rtpproxy" (or whatewer group rtpproxy belongs to) to read/write access to the controlling socket. Use-case - rtpproxy runs as rtpproxy user, while @sippy b2bua runs as b2bua user. If both of them are in rtpproxy group then with this patch b2bua can control rtpproxy just fine. Otherwise it cannot. See this RHBZ for further details:

https://bugzilla.redhat.com/626863

I believe this patch adds no visible changes for BSD users, although I'd love if someone will test it. So I think this is Linux-only related issue.