pgsql: Allow 'ELSEIF' as an alternative to 'ELSIF' in PL/PgSQL.

Log Message:
-----------
Allow 'ELSEIF' as an alternative to 'ELSIF' in PL/PgSQL. There have been
several reports of users being confused when they attempt to use ELSEIF
and run into trouble due to PL/PgSQL's lax parser. The parser will be
improved for 8.1, but we can fix most of the problem by allowing ELSEIF
for now.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.51&r2=1.52)
    pgsql/src/pl/plpgsql/src:
        scan.l (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.37&r2=1.38)