Various fixes, add renice command.

This commit adds the renice command and its man page,
it also introduces some fixes:
* Makes nice command more solid, it also makes it respect POSIX return values.
* Fixes estrtol, which produced a misleading error on out of range errors.
* Fixes chgrp.1 NAME section.

Signed-off-by: Christoph Lohmann <20h@r-36.net>

nice.c

@@ -1,38 +1,44 @@
 /* See LICENSE file for copyright and license details. */
-#include <unistd.h>
+#include <errno.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <errno.h>
-#include <string.h>
+#include <unistd.h>
 #include "util.h"
 
-static void
-usage(void)
-{
-	eprintf("usage: nice [-n inc] command [options ...]\n");
-}
+static void eusage(void);
 
 int
 main(int argc, char **argv)
 {
-	int inc = 10;
+	long val = 10;
 
 	ARGBEGIN {
 	case 'n':
-		inc = atoi(EARGF(usage()));
+		val = estrtol(EARGF(eusage()), 10);
 		break;
 	default:
-		usage();
+		eusage();
+		break;
 	} ARGEND;
 
-	nice(inc); /* POSIX specifies the nice failure still invokes the command. */
+	if(argc == 0)
+		eusage();
 
-	if(!*argv)
-		usage();
+	errno = 0;
+	nice((int)MAX(INT_MIN, MIN(val, INT_MAX)));
+	if(errno != 0)
+		perror("can't adjust niceness");
 
-	execvp(*argv, argv);
-	eprintf("nice: '%s': %s\n", *argv, strerror(errno));
-
-	return EXIT_FAILURE;
+	/* POSIX specifies the nice failure still invokes the command */
+	execvp(argv[0], argv);
+	/* reached only on failure */
+	perror(argv[0]);
+	return (errno == ENOENT)? 127 : 126;
 }
 
+static void
+eusage(void)
+{
+	eprintf("usage: nice [-n inc] command [options ...]\n");
+}