In particular, targets with weight 0 should be very UNlikely to be
selected, not very likely, as they were before. However, even ignoring
that bug in the logic, there was an additional bug (swapping list
items would cause the 0-weight items to get re-ordered incorrectly
anyway), and the code contained several fencepost errors.

This patch also adds gio/tests/srvtarget.c, which confirms that for a
sample list of targets, we now generate all possible correct random
sortings and no incorrect sortings, and the correct sortings occur in
roughly the expected proportions (though if the current code is
still wrong, those proportions may be wrong as well).

http://bugzilla.gnome.org/show_bug.cgi?id=583398