start_stop_daemon: add -N <nice> compat

[re]nice: add support for -nNNN w/o spaces, -NNN (nice only),
          simplified code

include/libbb.h

@@ -346,6 +346,7 @@ long xatol_sfx(const char *numstr, const struct suffix_mult *suffixes);
 long xatol(const char *numstr);
 /* Specialized: */
 unsigned xatou(const char *numstr);
+int xatoi_range(const char *numstr, int lower, int upper);
 int xatoi(const char *numstr);
 /* Using xatoi() instead of naive atoi() is not always convenient -
  * in many places people want *non-negative* values, but store them

include/usage.h

@@ -2691,6 +2691,7 @@ USE_FEATURE_MDEV_CONFIG( \
 USE_FEATURE_START_STOP_DAEMON_FANCY( \
 	"\n\t-o|--oknodo\t\t\texit status 0 if nothing done" \
 	"\n\t-v|--verbose\t\t\tbe verbose" \
+	"\n\t-N|--nicelevel <N>\t\tadd N to process's nice level" \
 ) \
 	"\n\t-s|--signal <signal>\t\tsignal to send (default TERM)" \
 	"\n\t-U|--chuid <username>|<uid>\tstart process with this name"