Add initial TLS (SSL) support to gio

This adds an extension point for TLS connections to gio, with a
gnutls-based implementation in glib-networking.

Full TLS support is still a work in progress; the current API is
missing some features, and parts of it may still be changed before
2.28.

https://bugzilla.gnome.org/show_bug.cgi?id=588189

gio/gio-marshal.list

@@ -27,3 +27,5 @@ INT:OBJECT
 VOID:INT64
 VOID:UINT64
 BOOLEAN:FLAGS
+BOOLEAN:OBJECT,FLAGS
+OBJECT:VOID