common: Hold reference to CockpitPipe while emitting signals

While emitting CockpitPipe::read signal, hold a reference to
self, in case the signal handler unrefs the pipe, we can still
act in a coherent manner.

Caught by Travis and Valgrind.
common: Backport g_unix_fd_add() and friends

Backport the new g_unix_fd_add() and related functions which
define GSource's for unix fds. This prevents us from having to
define a custom GSource in CockpitPipe.

In addition we'd like to use these sources for the reauthorization
work.