pgsql: Support SQL/PSM-compatible CASE statement in plpgsql.

Log Message:
-----------
Support SQL/PSM-compatible CASE statement in plpgsql.

Pavel Stehule

Modified Files:
--------------
pgsql/doc/src/sgml:
errcodes.sgml (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/errcodes.sgml?r1=1.23&r2=1.24)
plpgsql.sgml (r1.129 -> r1.130)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml?r1=1.129&r2=1.130)
pgsql/src/include/utils:
errcodes.h (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/errcodes.h?r1=1.24&r2=1.25)
pgsql/src/pl/plpgsql/src:
gram.y (r1.112 -> r1.113)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.112&r2=1.113)
pl_exec.c (r1.214 -> r1.215)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.214&r2=1.215)
pl_funcs.c (r1.71 -> r1.72)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c?r1=1.71&r2=1.72)
plerrcodes.h (r1.13 -> r1.14)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plerrcodes.h?r1=1.13&r2=1.14)
plpgsql.h (r1.99 -> r1.100)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.99&r2=1.100)
scan.l (r1.63 -> r1.64)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l?r1=1.63&r2=1.64)
pgsql/src/test/regress/expected:
plpgsql.out (r1.63 -> r1.64)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out?r1=1.63&r2=1.64)
pgsql/src/test/regress/sql:
plpgsql.sql (r1.53 -> r1.54)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql?r1=1.53&r2=1.54)