Add new MD5 pg_hba.conf keyword. Prevent fallback to crypt.

doc/src/sgml/client-auth.sgml

doc/src/sgml/jdbc.sgml

src/backend/libpq/auth.c

src/backend/libpq/hba.c

src/backend/libpq/pg_hba.conf.sample

src/include/libpq/hba.h