gdbus-codegen: Only generate autocleanup when instructed to

This adds a new --c-generate-autocleanup option to gdbus-codegen
which can be used to instruct gdbus-codegen about what autocleanup
definitions to emit.

Doing this unconditionally was found to interfere with existing
code out in the wild.

The new option takes an argument that can be
none, objects or all; to indicate whether to generate no
autocleanup functions, only do it for object types, or do it
for interface types as well. The default is 'objects', which
matches the unconditional behavior of gdbus-codegen on the 2.48
branch.

https://bugzilla.gnome.org/show_bug.cgi?id=763379