Make 'random' seeds cryptographically secure if possible
* configure.ac: Check for "/dev/urandom".
* src/sysdep.c (init_random) [HAVE_DEV_URANDOM]: Read the stream
for the seed from "/dev/urandom".
[WINDOWSNT]: Obtain the stream for the seed from w32 APIs.
* src/fns.c (Frandom): Update the doc string to indicate that
system entropy is used when available.
* src/w32.c: Include wincrypt.h.
(w32_init_crypt_random, w32_init_random): New functions, use the
CryptGenRandom API.
(globals_of_w32): Initialize w32_crypto_hprov handle to zero.
* src/w32.h (w32_init_random): Add prototype.
* doc/lispref/numbers.texi (Random Numbers): Document more details
about 't' as the argument to 'random'.
* etc/NEWS: Mention that '(random t)' now uses a cryptographically
strong seed if possible.