pgsql: Add an officially exported libpq function to encrypt passwords,

Log Message:
-----------
Add an officially exported libpq function to encrypt passwords, and
modify the previous \password patch to use it instead of depending
on a not-officially-exported function.  Per discussion.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.199 -> r1.200)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.199&r2=1.200)
    pgsql/src/bin/psql:
        command.c (r1.156 -> r1.157)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/command.c.diff?r1=1.156&r2=1.157)
    pgsql/src/bin/scripts:
        createuser.c (r1.24 -> r1.25)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/scripts/createuser.c.diff?r1=1.24&r2=1.25)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt.diff?r1=1.5&r2=1.6)
        fe-auth.c (r1.108 -> r1.109)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.108&r2=1.109)
        libpq-fe.h (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-fe.h.diff?r1=1.122&r2=1.123)