Relax g_thread_init() requirements

We now allow g_thread_init(NULL) to be called after other glib calls (with
some minor limitations). This is mainly a documentation change as this
really was already possible.

We also allow g_thread_init() to be called multiple times. Only the
first call actually initializes the threading system, further calls
are ignored (but print a warning if the argument is not NULL).

https://bugzilla.gnome.org/show_bug.cgi?id=606775