core: define C include for GIR

https://bugzilla.gnome.org/show_bug.cgi?id=740186
net: define C include for GIR

https://bugzilla.gnome.org/show_bug.cgi?id=740186
core: remove wrong (type uint) annotations for errors

For some reason, those annotations where added since daebdc455.
Today GIR seems to deal with callbacks errors fine, so let's remove it

https://bugzilla.gnome.org/show_bug.cgi?id=740186
core: annotate nullable error in callbacks

Since those arguments are optional error given to callbacks, annotate
them as nullable.

https://bugzilla.gnome.org/show_bug.cgi?id=740186
net: add GIR annotations for grl_net_wc_request_finish()

Annotate out arguments and the memory ownership.

https://bugzilla.gnome.org/show_bug.cgi?id=740186