Allow multiple-argument functions in constraint clauses.

 Formerly allowed only single arguments.
Declare column constraints using the usual list mechanism rather
 than explicit itemized lists.
Remove NOTNULL from default clause syntax (retain "NOT NULL").
 NOTNULL is not SQL92; eventually remove it from expressions too?
Move ISNULL, NOTNULL to Postgres-specific token declarations.
Fix up tabs and indenting on new CREATE USER commands.