Minor updates for date/time committed

I've committed some small changes to try to resolve the problem of
SQL99-mandated truncation of precision of time fields for TIMESTAMP and
TIME data types. Per the recent discussion, I've relaxed the mandated
behavior for unspecified precision of time fields for both date/time
literals and for schema definitions.

I've also committed some small changes to the ODBC driver to convert
CURRENT_TIME() (note the parens) to an ODBC-specific function per the
usual convention for supporting goofy ODBC functions.

I would *like to* remove the corresponding syntax extensions in gram.y
since they have little to no apparent value. However, the archives are
currently broken so I don't know if there are other reasons for wanting
them there.

- Thomas