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>

util/estrtol.c

@@ -13,13 +13,14 @@ estrtol(const char *s, int base)
 
 	errno = 0;
 	n = strtol(s, &end, base);
-	if(*end != '\0' || errno != 0) {
-		if(base == 0) {
+	if(*end != '\0') {
+		if(base == 0)
 			eprintf("%s: not an integer\n", s);
-		} else {
+		else
 			eprintf("%s: not a base %d integer\n", s, base);
-		}
 	}
+	if(errno != 0)
+		eprintf("%s:", s);
 
 	return n;
 }