Add new preprocessor directive syntax¸ using a '!' prefix.

Add new preprocessor directive syntax¸ using a '!' prefix.

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.