test_mmap wrote null bytes into its expected-output file; this caused me to

test_mmap wrote null bytes into its expected-output file; this caused me to
waste an hour tracking down an illusion; repaired it; writing/reading non-
printable characters (except \t\r\n) into/outof text-mode files ain't
defined x-platform, and at least some Windows text editors do surprising
things in their presence.
Also added a by-hand "build humber" to the Windows build, in an approximation
of Python's inexplicable BUILD-number Unix scheme.  I'll try to remember to
increment it each time I make a Windows installer available.  It's starting
at 2, cuz I've put 2 installers out so far (both with BUILD #0).