cobol: Accept device names as filenames and CR in alphanumeric constants.

SYSIN is not a reserved word per IBM.  Therefore, it may be a
user-defined name. For now, at least accept it as a filename.

Modify lexer to accept CR in a string constant.  Technically string
literals should not contain control codes, but in existing practice
they do.  gcobol now accepts any value other than NL and NUL.

gcc/cobol/ChangeLog:

	* parse.y: Accept device name as filename in SELECT.
	* scan.l: Remove CR exclusion and rename STRING to STRING2.