GOption: add strict posix mode

Add a "posixly correct" mode to GOption to stop parsing arguments as
soon as the first non-option argument is encountered.

We determine the default value on the basis of duplicating the behaviour
of the system getopt() implementation (which we directly check the
behaviour of at runtime).  On GNU systems this allows the user to modify
our behaviour using POSIXLY_CORRECT.

The user can change the value by g_option_context_set_strict_posix(),
which might be useful for some usecases of GOptionContext (as mentioned
in the doc string of this new function).

https://bugzilla.gnome.org/show_bug.cgi?id=723160