Linux sockets should maybe pass effective UID/GID

For non-Linux sockets, libqb appears to use the effective UID/GID as the socket credentials, and given the member naming in struct qb_ipcc_connection (i.e. euid/egid), it seems that was the original intent as well.

However, for Linux sockets, libqb enables SO_PASSCRED when connecting, and qb_ipc_auth_creds() pulls the other side's credentials from SCM_CREDENTIALS. I believe libqb on the other side lets the credentials default, which the kernel will set to the user's real UID/GID. I think only root can set SCM_CREDENTIALS when connecting, so it might not be helpful in all cases, but it would nice to use the effective UID/GID when possible.

For context, see this mailing list thread:
https://www.mail-archive.com/users@clusterlabs.org/msg10681.html