pgsql: Re-defines SHA2 symbols so that they would not conflict with

Log Message:
-----------
Re-defines SHA2 symbols so that they would not conflict with certain
versions of OpenSSL. If your OpenSSL does not contain SHA2, then there
should be no conflict. But ofcourse, if someone upgrades OpenSSL,
server starts crashing.

Backpatched to 8.1.X.

Marko Kreen

Modified Files:
--------------
pgsql/contrib/pgcrypto:
sha2.c (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/sha2.c.diff?r1=1.5&r2=1.6)
sha2.h (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/sha2.h.diff?r1=1.2&r2=1.3)