A directive that contains an unterminated """ string should not
cause each word of the string to be treated as a bare word. Instead,
the directive should fail to parse.
There are two tests. One just checks that a complete directive
containing such a string fails to parse. The other checks for a case
where a directive ends with a very long unterminated """ string,
and the directive is itself not closed. While this test won't fail,
it does trigger a nasty perl warning.
Add a prefix_directives option to the setup file to turn this syntax
on; currently defaults to false, for backward compatibility. Support
optional '!' prefix even with prefix_directives off, and use that in
the underlay to support either setting of prefix_directives. Add NEWS
entry with migration information.