scanner: don't allow an unknown mark to run into a string or comment

scanner: don't allow an unknown mark to run into a string or comment

This means that e.g.
   printf("hello")
where no marks are declared will not treat
    ("
as an known mark, but instead find
    (
and then a string.

This is important for skipping over C code in 'parsergen'.

Signed-off-by: NeilBrown <neilb@suse.de>