Clean up process of calling g_debug_init()

Clean up process of calling g_debug_init()

Make sure that it calls absolutely nothing that may ever recurse back
into GLib again:

  - g_ascii_strcasecmp() is unsafe because it has g_return_if_fail() at
    the top.  As far as I know, the only ASCII character letter that
    would get special treatment here is "i" and that appears in neither
    "help" nor "all".

  - g_getenv() is very complicated on Windows, so use a simple version
    that is sufficient for our purposes.

Now that it's completely safe, we can just call it from g_logv() in the
usual way without all of the hacks.

https://bugzilla.gnome.org/show_bug.cgi?id=660744