Add support for RADIUS authentication.

doc/src/sgml/client-auth.sgml

src/backend/libpq/auth.c

src/backend/libpq/hba.c

src/backend/libpq/md5.c

src/backend/libpq/pg_hba.conf.sample

src/include/libpq/hba.h

src/include/libpq/md5.h