pgsql: Have TRUNCATE advance the affected table's relfrozenxid to

Log Message:
-----------
Have TRUNCATE advance the affected table's relfrozenxid to RecentXmin, to
avoid a later needless VACUUM for Xid-wraparound purposes.  We can do this
since the table is known to be left empty, so no Xid remains on it.

Per discussion.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        index.c (r1.282 -> r1.283)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.282&r2=1.283)
    pgsql/src/backend/commands:
        tablecmds.c (r1.223 -> r1.224)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.223&r2=1.224)
    pgsql/src/include/catalog:
        index.h (r1.73 -> r1.74)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/index.h.diff?r1=1.73&r2=1.74)