pgsql: Fix the just-reported problem that you can't specify all four

Log Message:
-----------
Fix the just-reported problem that you can't specify all four trigger event
types in CREATE TRIGGER.  While at it, clean up the amazingly tedious and
inextensible way that the trigger event type list was handled.  Per report
from Greg Sabino Mullane.

Modified Files:
--------------
    pgsql/src/backend/commands:
        tablecmds.c (r1.287 -> r1.288)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.287&r2=1.288)
        trigger.c (r1.247 -> r1.248)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c?r1=1.247&r2=1.248)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.431 -> r1.432)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.431&r2=1.432)
        equalfuncs.c (r1.354 -> r1.355)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.354&r2=1.355)
    pgsql/src/backend/parser:
        gram.y (r2.664 -> r2.665)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.664&r2=2.665)
    pgsql/src/include/nodes:
        parsenodes.h (r1.394 -> r1.395)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.394&r2=1.395)