GProxyAddressEnumerator: add default-port property

Although none of the in-tree GSocketConnectable types need it, other
types (like SoupAddress) may find it useful to be able to pass a URI
and a default-port to GProxyAddressEnumerator separately (the same way
you can with GNetworkAddress). So add a default-port property.

https://bugzilla.gnome.org/show_bug.cgi?id=698877
Remove a preprocessed-out code section

If it has not been needed since 2000, there probably is no use for it.
Moved the G_MAIN_POLL_DEBUG stuff to gmain-internal.h

Now that code is about to drift from gmain.c into the poll backend,
the debug infrastructure should better be shared in the header.
I'm too lazy to deduplicate it in gpoll.c, though.
Started factoring the poll backend out of GMainContext
Factored out the poll-centric API support functions

The methods of GMainContext that assume it is backed by a GPollFunc
will be supported only by the poll backend. The context will switch
to the poll backend to complete any of these functions.
Provide a user data pointer to custom main contexts

It could be used to pass a uv_loop_t* for the libuv backend,
for example.
Added acquire and release functions for main loop backends
Added g_main_context_check_sources()

It takes the part of g_main_context_check() that is useful
to main loop backends, sans the fd array.
Factored iterate and poll out into the backend