Previously, if a method was not annotated with org.gtk.GDBus.C.UnixFD
then the generated code would never contain GUnixFDList parameters, even
if the method has 'h' (file descriptor) parameters. However, in this
case, the generated code is essentially useless: the method cannot be
called or handled except in degenerate cases where the file descriptors
are missing or ignored.

Check the argument types for 'h', and if present, generate code as if
org.gtk.GDBus.C.UnixFD annotation were specified.

This change will break any existing code which refers to the (useless)
wrappers for such methods. The workaround for such code is to add the
org.gtk.GDBus.C.UnixFD annotation, which will cause the same generated
code to be emitted before and after this change.

If this is found to cause widespread problems, we can explore a
different approach (perhaps emitting a warning from the code generator,
or annotating the symbols as deprecated).

#1726