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>

renice.c

@@ -0,0 +1,117 @@
+/* See LICENSE file for copyright and license details. */
+#include <errno.h>
+#include <limits.h>
+#include <pwd.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/resource.h>
+#include "util.h"
+
+static int strtop(const char *);
+static bool renice(int, int, long);
+static void eusage(void);
+
+int
+main(int argc, char **argv)
+{
+	const char *adj = NULL;
+	long val;
+	int i, which = PRIO_PROCESS, status = 0;
+
+	ARGBEGIN {
+	case 'n':
+		adj = EARGF(eusage());
+		break;
+	case 'g':
+		which = PRIO_PGRP;
+		break;
+	case 'p':
+		which = PRIO_PROCESS;
+		break;
+	case 'u':
+		which = PRIO_USER;
+		break;
+	default:
+		eusage();
+		break;
+	} ARGEND;
+
+	if(argc == 0 || !adj)
+		eusage();
+
+	val = estrtol(adj, 10);
+	for(i = 0; i < argc; i++) {
+		int who = -1;
+
+		if(which == PRIO_USER) {
+			const struct passwd *pwd;
+
+			errno = 0;
+			do pwd = getpwnam(argv[i]); while(errno == EINTR);
+
+			if(pwd)
+				who = pwd->pw_uid;
+			else if(errno != 0) {
+				perror("can't read passwd");
+				status = 1;
+				continue;
+			}
+		}
+		if(who < 0)
+			who = strtop(argv[i]);
+
+		if(who < 0 || !renice(which, who, val))
+			status = 1;
+	}
+
+	return status;
+}
+
+static int
+strtop(const char *s)
+{
+	char *end;
+	long n;
+
+	errno = 0;
+	n = strtol(s, &end, 10);
+	if(*end != '\0') {
+		fprintf(stderr, "%s: not an integer\n", s);
+		return -1;
+	}
+	if(errno != 0 || n <= 0 || n > INT_MAX) {
+		fprintf(stderr, "%s: invalid value\n", s);
+		return -1;
+	}
+
+	return (int)n;
+}
+
+static bool
+renice(int which, int who, long adj)
+{
+	errno = 0;
+	adj += getpriority(which, who);
+	if(errno != 0) {
+		fprintf(stderr, "can't get %d nice level: %s\n",
+		        who, strerror(errno));
+		return false;
+	}
+
+	adj = MAX(PRIO_MIN, MIN(adj, PRIO_MAX));
+	if(setpriority(which, who, (int)adj) == -1) {
+		fprintf(stderr, "can't set %d nice level: %s\n",
+		        who, strerror(errno));
+		return false;
+	}
+
+	return true;
+}
+
+static void
+eusage(void)
+{
+	eprintf("renice -n inc [-g | -p | -u] ID ...\n");
+}