Few cleanups and couple of new things:
- add SHA2 algorithm to older OpenSSL
- add BIGNUM math to have public-key cryptography work
on non-OpenSSL build.
- gen_random_bytes() function
The status of SHA2 algoritms and public-key encryption
can now be changed to 'always available.'
That makes pgcrypto functionally complete and unless there
will be new editions of AES, SHA2 or OpenPGP standards,
there is no major changes planned.
