pgsql: Add ALTER TABLE ENABLE/DISABLE TRIGGER commands.

Log Message:
-----------
Add ALTER TABLE ENABLE/DISABLE TRIGGER commands.  Change pg_dump to
use these instead of its previous hack of changing pg_class.reltriggers.
Documentation is lacking, will add that later.
Patch by Satoshi Nagayasu, review and some extra work by Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/commands:
        tablecmds.c (r1.168 -> r1.169)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.168&r2=1.169)
        trigger.c (r1.192 -> r1.193)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.192&r2=1.193)
    pgsql/src/backend/parser:
        gram.y (r2.507 -> r2.508)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.507&r2=2.508)
        keywords.c (r1.164 -> r1.165)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/keywords.c.diff?r1=1.164&r2=1.165)
    pgsql/src/bin/pg_dump:
        pg_backup_archiver.c (r1.113 -> r1.114)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_archiver.c.diff?r1=1.113&r2=1.114)
        pg_dump.c (r1.418 -> r1.419)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.418&r2=1.419)
        pg_dump.h (r1.119 -> r1.120)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.h.diff?r1=1.119&r2=1.120)
    pgsql/src/include/commands:
        trigger.h (r1.54 -> r1.55)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h.diff?r1=1.54&r2=1.55)
    pgsql/src/include/nodes:
        parsenodes.h (r1.289 -> r1.290)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.289&r2=1.290)