ps: exit with exit status 1 if no processes shown.

This matches procps ps, and at least one LTP test relies on it. The
behavior is specifically "nothing matches", not "one PID didn't match":
assuming no process 666, `ps 666` exits with 1, but `ps 1 666` exits
with 0.

toys/posix/ps.c

@@ -1400,6 +1400,7 @@ void ps_main(void)
     if (CFG_TOYBOX_FREE) free(tbsort);
   }
 
+  if (!TT.kcount) toys.exitval = 1;
   if (CFG_TOYBOX_FREE) {
     free(TT.gg.ptr);
     free(TT.GG.ptr);