Refactor: libcrmcommon: make crm_ipc_get_fd() a wrapper for new internal function

The new function, pcmk__ipc_fd(), returns a standard Pacemaker return code and
doesn't log errors.

Note that the new function doesn't need to call qb_ipcc_fd_get() because it is
only valid with a connected IPC object, which will have already called that to
load the file descriptor into crm_ipc_t:pfd.fd.