Use PRIO_{MIN,MAX} instead of NZERO

Some systems do not define NZERO.  NZERO seems to be an XSI extension
and PRIO_{MIN,MAX} is not specified by POSIX but most relevant systems
define it.

nice.c

@@ -37,7 +37,7 @@ main(int argc, char *argv[])
 	val += getpriority(PRIO_PROCESS, 0);
 	if (errno != 0)
 		weprintf("getpriority:");
-	val = MAX(-NZERO, MIN(val, NZERO - 1));
+	val = MAX(PRIO_MIN, MIN(val, PRIO_MAX));
 	if (setpriority(PRIO_PROCESS, 0, val) != 0)
 		weprintf("setpriority:");
 

renice.c

@@ -105,7 +105,7 @@ renice(int which, int who, long adj)
 		return 0;
 	}
 
-	adj = MAX(-NZERO, MIN(adj, NZERO - 1));
+	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));