Working draft changes for C99 preprocessor synchronization

Synchronizes the preprocessor with C99: variadic macros, _Pragma, mixed declarations. Adopted with two changes: make the 16.8p3 change normative rather than a note ('Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore'); omit 'preprocessing' in 'preprocessing translation unit' in the change to 16.3.5p1 (J16 19-0-0, WG21 8-0-0).