Fix some easy compiler warnings

make ggp_date_from_iso8601 use a GDateTime instead of GTimeVal and have it return a gint64

Wrap a signal callback in a G_CALLBACK like it should be

Remove some unused variables

Only include sysctl.h on platforms where we need it. It's deprecated under gcc now.

Testing Done:
Compiled on freebsd 12.0, netbsd 9.0, openbsd 6.7, mac, and linux without issues.

Reviewed at https://reviews.imfreedom.org/r/69/