ipc: add qb_ipcc_auth_get() API call (#418)

• ipc: addd qb_ipcc_auth_get() API call

We can't use SO_PEERCRED on the client fd when using socket IPC
becayse it's a DGRAM socket (pacemaker tries this). So provide
an API to get the server credentials that libqb has already
squirreled away for its own purposes.

Also, fix some unused-variable compiler warnings in unix.c
when building on systems without posix_fallocate().