gmain: Eliminate use of ABS from g_timeout_set_expiration()

It’s not clear why `ABS()` was ever needed here so, as per the previous
commit, let’s eliminate it.

It was added in d9f5ab56c3 owing to the need for a pseudo-random value
in the range (0, 1000000). Previously the code calculated a hash
(unsigned pseudo-random value in the full range of `unsigned int`), then
cast it to `int` (making half its possible values negative), then took
`ABS()` of that (forcing the negative values positive, so effectively
halving the pseudo-random state space and doubling the possibility of
any particular value being chosen), then did mod 1000000 to limit the
upper bound.

We can simplify this by eliminating the cast to `int`, which removes the
need for `ABS()`. The whole `unsigned int` state space is then passed to
mod 1000000. This should give the same probability distribution as
before.

The `timer_perturb` variable itself is signed to allow for `-1` to
indicate it’s not been set yet (it’s `static`). This could be split out
into a separate boolean, but there’s no point as the desired
`timer_perturb` state space is only (0, 1000000) anyway, which is easily
representable as a signed `int`.

Spotted by Thomas Haller in
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/4559#note_2384322.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/gmain.c

@@ -5056,8 +5056,7 @@ g_timeout_set_expiration (GTimeoutSource *timeout_source,
             session_bus_address = g_getenv ("HOSTNAME");
           if (session_bus_address)
             {
-              int hash = g_str_hash (session_bus_address);
-              timer_perturb = ABS (hash) % 1000000;
+              timer_perturb = (int) (g_str_hash (session_bus_address) % 1000000);
             }
           else
             {