Make prayer timeouts longer in very long games

This is a security fix, intended to avoid a DoS attack (that has
been used in practice against public servers) in which a situation
is created in which the same short loop of commands can be run
over and over indefinitely; this causes a lot of CPU usage on the
public server as it simulates the game turns, and requires hardly
any network or CPU usage for an attacker.

Once the turn counter goes above 100000, the average prayer timeout
increases by 1 for every additional 100 turns spent. Thus, even
with a ring of slow digestion, nutrition prayers will no longer
be possible after turn 2 million or so. This is unlikely to have
any noticeable effect on any strategy that does not rely heavily
on prayer for nutrition; even in an implausible 200000-turn game,
prayer timeout will only be increased by 1000 or so, which should
be easily manageable for a character that powerful.

src/pray.c

@@ -1217,6 +1217,14 @@ aligntyp g_align;
     if (kick_on_butt)
         u.ublesscnt += kick_on_butt * rnz(1000);
 
+    /* Avoid games that go into infinite loops of copy-pasted commands with no
+       human interaction; this is a DoS vector against the computer running
+       NetHack. Once the turn counter is over 100000, every additional 100 turns
+       increases the prayer timeout by 1, thus eventually nutrition prayers will
+       fail and some other source of nutrition will be required. */
+    if (g.moves > 100000L)
+        u.ublesscnt += (g.moves - 100000L) / 100;
+
     return;
 }