truncate cleanup

The attached patch cleans up the implementation of the TRUNCATE command;
in the current code, the authentication logic (check user, check the
relation we're operating on, etc) is done in tcop/utility.c, whereas the
actual TRUNCATE command in done in TruncateRelation() in
commands/createinh.c (which is really just a wrapper over
heap_truncate() in catalog/heap.c). This patch moves the authentication
logic into TruncateRelation(), as well as making some minor code
cleanups.

FWIW, I'm not really sure why TruncateRelation() is in
commands/createinh.c to begin with; if there is a preference, I can
remove TruncateRelation() altogether and move everything into
heap_truncate().

Cheers,

Neil

--
Neil Conway <neilconway@rogers.com>
PGP Key ID: DB3C29FC