Adding the new C files too.

nice.c

@@ -0,0 +1,38 @@
+/* See LICENSE file for copyright and license details. */
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <string.h>
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: nice [-n inc] command [options ...]\n");
+}
+
+int
+main(int argc, char **argv)
+{
+	int inc = 10;
+
+	ARGBEGIN {
+	case 'n':
+		inc = atoi(EARGF(usage()));
+		break;
+	default:
+		usage();
+	} ARGEND;
+
+	nice(inc); /* POSIX specifies the nice failure still invokes the command. */
+
+	if(!*argv)
+		usage();
+
+	execvp(*argv, argv);
+	eprintf("nice: '%s': %s\n", *argv, strerror(errno));
+
+	return EXIT_FAILURE;
+}
+