fix MSVC build
sha1: moved wrapper SHA1() to sha1.c
Introduced PRNG based on Hash_DRBG (NIST SP 800-90A)

This pseudo-random number generator solves problem with
platform-independent generation of randomized nonces.
Current implementation uses weak entropy, especially when
kernel.randomize_va_space = 0. But it can be improved by
adding new sources to xmpp_rand_reseed().

New internal API introduced:
  xmpp_rand_new
  xmpp_rand_free
  xmpp_rand
  xmpp_rand_bytes
  xmpp_rand_nonce