cobol: Accept ASSIGN TO DEVICE.

For SELECT fd-name ASSIGN TO dev, "dev" may name either a runtime
environment variable (as previously) or, as now, an
implementation-defined device.  New warnings govern dialect
variations.  Fixes RT 3617.

gcc/cobol/ChangeLog:

	* cbldiag.h (enum cbl_diag_id_t): New warnings.
	* cobol1.cc (cobol_langhook_handle_option): Process warnings.
	* gcobol.1: Document warnings and devices.
	* lang-specs.h: Accept warngings.
	* lang.opt: Define warnings.
	* messages.cc: Associate warnings with dialects.
	* parse.y: Parse new syntax.