Update sequence-related functions to new fmgr style. Remove downcasing,

Update sequence-related functions to new fmgr style.  Remove downcasing,
quote-stripping, and acl-checking tasks for these functions from the
parser, and do them at function execution time instead.  This fixes
the failure of pg_dump to produce correct output for nextval(Foo)
used in a rule, and also eliminates the restriction that the argument
of these functions must be a parse-time constant.