pgsql: In PL/PgSQL, allow a block's label to be optionally specified at

Log Message:
-----------
In PL/PgSQL, allow a block's label to be optionally specified at the
end of the block:

<<label>>
begin
    ...
end label;

Similarly for loops. This is per PL/SQL. Update the documentation and
add regression tests. Patch from Pavel Stehule, code review by Neil
Conway.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.74&r2=1.75)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.78&r2=1.79)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.37&r2=1.38)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.32&r2=1.33)