Makefile.PL: update copyright date
Unstable release 0.016
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.
Release 0.017
Fix registering unregistered enums
Mark a few tests as known to fail

Some ref counting tests fail due to a recent change in perl-Glib:
<https://git.gnome.org/browse/perl-Glib/commit/?id=fa876ff132035f1b5cc96428bd0a75ea939aa295>.