g_key_file_has_key_full: New function to fix g_key_file_has_key()'s GError semantics

See https://bugzilla.gnome.org/show_bug.cgi?id=649657 for discussion
of why it's bad for bindings for gerror return values to both signal
errors and carry meaning.

https://bugzilla.gnome.org/show_bug.cgi?id=650345