gsource: Add support for file descriptors on UNIX

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