Fix PRNG seeding to resist guessing

Fix PRNG seeding to resist guessing

We seed it with value of time().  It's the traditional way, but it
provides only a few bits of effective entropy when an attacker has a
rough idea when the program started.

Instead, seed with a kernel random number.  If we can't get one, fall
back to a hash of gettimeofday() and getpid().  This should happen
only on old systems or Windows.  Far worse than a kernel random
number, but far better than using time().

Note that fairland used to seed with time() + getpid() until commit
331aac2a (v4.2.20) dropped the getpid(), claiming it didn't improve
the randomness.  Perhaps it didn't under Windows then, but it
certainly did elsewhere, so it was a regression.