Make the $@ macro behave as described in the man page.

This patch has been submitted by Andreas Burmester (i think) on the
German Usenet long ago.  Andreas told me that our m4 has quite more
subtle bugs, so we might consider using NGU m4 instead.

usr.bin/m4/eval.c

@@ -421,6 +421,16 @@ register int argc;
 				}
 				pbstr(argv[2]);
 				break;
+			case '@':
+				for( n = argc - 1; n >= 2; n-- )
+				{
+					putback(rquote);
+					pbstr(argv[n]);
+					putback(lquote);
+					if( n > 2 )
+						putback(',');
+				}
+				break;
 			default:
 				putback(*p);
 				putback('$');