gsource: Add support for file descriptors on UNIX

Adding file descriptors to a GSource provides similar functionality to
the old g_source_add_poll() API with two main differences.

First: the list of handles is managed internally and therefore users are
prevented from randomly modifying the ->events field.  This prepares us
for an epoll future where changing the event mask is a syscall.

Second: keeping the list internally allows us to check the ->revents for
events for ourselves, allowing the source to skip implementing
check/prepare.  This also prepares us for the future by allowing an
implementation that doesn't need to iterate over all of the sources
every time.

https://bugzilla.gnome.org/show_bug.cgi?id=686853

docs/reference/glib/glib-sections.txt

@@ -530,6 +530,10 @@ GSourceFunc
 g_source_set_callback_indirect
 g_source_set_ready_time
 g_source_get_ready_time
+g_source_add_unix_fd
+g_source_remove_unix_fd
+g_source_modify_unix_fd
+g_source_query_unix_fd
 g_source_add_poll
 g_source_remove_poll
 g_source_add_child_source