Remove handrolled strcmp()'s

Favor readability over bare-metal.

sed.c

@@ -1114,7 +1114,7 @@ next_file(void)
 			if (first) /* given no files, default to stdin */
 				file = stdin;
 			/* else we've used all our files, leave file = NULL */
-		} else if ((*files)[0] == '-' && !(*files)[1]) {
+		} else if (!strcmp(*files, "-")) {
 			file = stdin;
 			files++;
 		} else if (!(file = fopen(*files++, "r"))) {