Allow default expressions to be attached to columns of foreign tables.

 Allow default expressions to be attached to columns of foreign tables.
 
 There's still some discussion about exactly how postgres_fdw ought to
 handle this case, but there seems no debate that we want to allow defaults
 to be used for inserts into foreign tables.  So remove the core-code
 restrictions that prevented it.
 
 While at it, get rid of the special grammar productions for CREATE FOREIGN
 TABLE, and instead add explicit FEATURE_NOT_SUPPORTED error checks for the
 disallowed cases.  This makes the grammar a shade smaller, and more
 importantly results in much more intelligible error messages for
 unsupported cases.  It's also one less thing to fix if we ever start
 supporting constraints on foreign tables.