Allow "-" to be specified as an operand as well as an option.

SUSV3 requires something like "split -- -" to work. Document the "-" operand.

usr.bin/split/split.1

@@ -50,8 +50,14 @@ The
 .Nm
 utility reads the given
 .Ar file
-(or standard input if no file is specified)
 and breaks it up into files of 1000 lines each.
+If
+.Ar file
+is a single dash
+.Pq Ql \&-
+or absent,
+.Nm
+reads from the standard input.
 .Pp
 The options are as follows:
 .Bl -tag -width Ds

usr.bin/split/split.c

@@ -142,7 +142,9 @@ main(argc, argv)
 
 	if (*argv != NULL)
 		if (ifd == -1) {		/* Input file. */
-			if ((ifd = open(*argv, O_RDONLY, 0)) < 0)
+			if (strcmp(*argv, "-") == 0)
+				ifd = STDIN_FILENO;
+			else if ((ifd = open(*argv, O_RDONLY, 0)) < 0)
 				err(EX_NOINPUT, "%s", *argv);
 			++argv;
 		}