gdbus-codegen: emit GUnixFDLists if an arg has type 'h'

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).

GNOME/glib#1726
10 jobs for 1726-gdbus-codegen-h in 30 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #415670
cross-android_api21_arm64

00:02:03

passed #415671
cross-android_api28_arm64

00:01:22

passed #415672
cross-mingw64

00:02:21

passed #415668
debian-stable-x86_64

00:02:29

passed #415667
fedora-x86_64

00:06:23

passed #415669
G_DISABLE_ASSERT

00:02:14

passed #415673
win32
msys2-mingw32

00:14:37

passed #415674
win32
vs2017-x64

00:09:57

 
  Coverage
passed #415675
coverage

00:01:55

77.8%
 
  Analysis
passed #415676
scan-build

00:13:28