cobol: Relax parsing restrictions for void sentences and MF $END-IF.

Accept multiple consecutive periods as empty statements.  Accept
undocumented $END-IF under -dialect mf without comment.

gcc/cobol/ChangeLog:

	PR cobol/126495
	PR cobol/126496
	* gcobol.1: Update manual to reflect new syntax.
	* parse.y: Allow void sentence and increment s/r expected count to 7.
	* scan.l: Lex $END-IF, optionally, as $END.