cobol: Replace NUL input with space, with diagnostic.

Old-school COBOL input may embed NUL characters in alphanumeric
literals. GNU Flex unfortunately treats NUL as end-of-file marker.

Detect the situation in the lexio file-reader.  Replace NUL with
SPACE, increment error count, and emit message. Fixes RT 3622.

	* lexio.h (parse_error_inc): Declare function.
	(struct filespan_t::next_line): Detect and remove NUL,