Fix compiler warnings related to format specifiers
Register unregistered enums

Using the introspection information, create a custom GType for unregistered
enums/flags and register it with perl-Glib.

Relevant for, e.g., GSpawnFlags and VtePtyFlags.
Pass on NULL user data in C invocations if appropriate

If the user did not specify any callback and user data, and if there is no slot
for a C destroy notify function pointer, then pass on NULL for the C user data.
There is no point in passing on anything else, and some libraries (e.g., vte)
even require that the C user data be NULL if the C function pointer is NULL.