pgsql: Allow the parameters to PL/PgSQL's RAISE statement to be

Log Message:
-----------
Allow the parameters to PL/PgSQL's RAISE statement to be expressions,
instead of just scalar variables. Add regression tests and update the
documentation. Along the way, remove some redundant error checking
code from exec_stmt_perform().

Original patch from Pavel Stehule, reworked by Neil Conway.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.71 -> r1.72)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.71&r2=1.72)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.75&r2=1.76)
        pl_exec.c (r1.143 -> r1.144)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.143&r2=1.144)
        pl_funcs.c (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.42&r2=1.43)
        plpgsql.h (r1.62 -> r1.63)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.62&r2=1.63)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.33&r2=1.34)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.28&r2=1.29)