parsergen: require all terms to be declared if any are.

parsergen: require all terms to be declared if any are.

The previous implementation only required all terms used for the first
time after a $TERM declaration to be declared.  That isn't really
useful.

So change it to only set the default symbol type of Term after all the
grammar is parsed, and then only if not $TERM declarations were found.

This highlights that various terminals in ocean weren't declared, so
declare them now.

Signed-off-by: NeilBrown <neil@brown.name>