tweak CREATE SEQUENCE grammar

This patch makes a few minor changes to the parsing of CREATE SEQUENCE
to allow for some syntax variations consistent with SQL 2002 (or
whatever the official name of the current draft standard will be),
section 11.62:

START -> START [ WITH ]
INCREMENT -> INCREMENT [ BY ]
CYCLE -> [ NO ] CYCLE

Should this new syntax be the default? Being standards-compliant is
nice, and IMHO, the new syntax is nicer, anyway. But I'm not too
worried about that, and I haven't encouraged the new syntax in the
documentation.

BTW, there are a few other small changes needed, as well as the
addition of ALTER SEQUENCE, but other than that the new "sequence
generator" feature of the standard is almost exactly what we've always
implemented, AFAICT.

This should be saved for 7.4, please.

Cheers,

Neil

--
Neil Conway <neilc(at)samurai(dot)com> || PGP Key ID: DB3C29FC