gdbus: Add FD support for gdbus call

Gdbus call could not take file handle (parameter 'h') as a parameter.

Original patch from Tim Waugh <twaugh@redhat.com>.

Fixes: #961

docs/reference/gio/gdbus.xml

@@ -321,6 +321,18 @@ $ gdbus call --session \
              5000
 (uint32 12,)
 </programlisting>
+<para>
+  Call a method with file handle argument:
+</para>
+<programlisting>
+$ gdbus call --session \
+             --dest org.example.foo \
+             --object-path /org/example/foo \
+             --method SendFDs \
+             1 \
+             10 \
+             10&lt;file.foo
+</programlisting>
 <para>
   Monitoring all objects on a service:
 </para>