gsocket: use cancellable child source
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

tchakabam/glib,ieei/glib,Distrotech/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,cention-sany/glib,ieei/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,mzabaluev/glib,mzabaluev/glib,krichter722/glib,johne53/MB3Glib,gale320/glib,cention-sany/glib,mzabaluev/glib,krichter722/glib,tchakabam/glib,cention-sany/glib,krichter722/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,ieei/glib,gale320/glib,tamaskenez/glib,johne53/MB3Glib,mzabaluev/glib,endlessm/glib,tamaskenez/glib,lukasz-skalski/glib,johne53/MB3Glib,lukasz-skalski/glib,Distrotech/glib,gale320/glib,tchakabam/glib,lukasz-skalski/glib,Distrotech/glib,krichter722/glib,ieei/glib,endlessm/glib,gale320/glib,endlessm/glib,tamaskenez/glib,tamaskenez/glib,cention-sany/glib,ieei/glib,Distrotech/glib,cention-sany/glib,mzabaluev/glib,Distrotech/glib,endlessm/glib,gale320/glib,krichter722/glib,endlessm/glib,tchakabam/glib,MathieuDuponchelle/glib,johne53/MB3Glib,tamaskenez/glib,MathieuDuponchelle/glib,tchakabam/glib