gmain: repeat preconditions for emphasis

g_main_context_acquire() mentions that you must have called it before
you make any calls to _prepare(), _query(), _check() or _dispatch().

For emphasis, add a note on each of those functions pointing back to the
fact that you must have called _acquire() before using them.
gsocket: trivial typo fix

https://bugzilla.gnome.org/show_bug.cgi?id=724707
gsocket: use cancellable child source

Now that GCancellable's GSource is based on _set_ready_time() instead of
an fd, we should use it as a child source, instead of forcing the
creation of the fd and adding it as a poll.

https://bugzilla.gnome.org/show_bug.cgi?id=724707
gsocket: don't abuse GPollFD.revents field

We are reusing the GPollFD.revents field of the source to store a
temporary value.  Use a local variable for that instead.

This is a refactor to make the next commit easier to understand.

https://bugzilla.gnome.org/show_bug.cgi?id=724707
GSource: mark some API as "implementation only"

Clarify that _add_poll() _remove_poll() _add_unix_fd(),
_modify_unix_fd(), _remove_unix_fd(), _query_unix_fd(),
_set_ready_time(), _add_child_source() and _remove_child_source() are only
intended to be used by the implementation of a particular GSource -- not its
consumers.

https://bugzilla.gnome.org/show_bug.cgi?id=724707
gsocket: make use of g_source_set_ready_time()

Drop our own hand-rolled version of the same functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=724707
gsocket: use _add_unix_fd() instead of _add_poll()

Use g_source_add_unix_fd() on UNIX instead of using a GPollFD.

https://bugzilla.gnome.org/show_bug.cgi?id=724707
gsocket: use check/prepare only on win32

There is no longer any code left in the check/prepare functions on UNIX,
so put %NULL in the GSourceFuncs vtable.

This also allows us to simplify some logic.

https://bugzilla.gnome.org/show_bug.cgi?id=724707