Switch PRNG from BSD random() to Mersenne Twister

random() may yield different pseudo-random number sequences for the
same seed on another system.  For instance, at least some versions of
MinGW provide a random() in -liberty that differs from traditional BSD
(see commit c8231b12).  Rather inconvenient for regression testing.

MT19937 Mersenne Twister is a proven, high-quality PRNG.  Actual code
is reference code provided by the inventors[*].  Quick tests show
performance comparable to random().

Like random(), MT is not cryptographically secure: observing enough of
its output permits guessing its state, and thus its future output.  I
don't think players can do that.

Drop the copy of BSD random() we added for Windows.

Like the previous commit, this changes the server's die rolls, and
makes fairland create a different random map for the same seed.  Update
expected smoke test results accordingly.

[*] mt19937ar.sep.tgz downloaded from
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/MT2002/emt19937ar.html

src/lib/subs/journal.c

@@ -191,7 +191,7 @@ journal_reopen(void)
 void
 journal_prng(unsigned seed)
 {
-    journal_entry("prng BSD %d", seed);
+    journal_entry("prng MT19937 %u", seed);
 }
 
 void