Fix #3235

. diag_nanobsd.php:
 . Since conf_mount_ro() is always being called, always call
   conf_mount_rw to keep refcount correct
 . Do not show refcount_read() return when it's -1
. config.lib.inc
 . Increment and decrement refcount even if nanobsd_force_rw is set.
   This is necessary to avoid partition being mounted as RO when
   nanobsd_force_rw is disabled and any process requested it to be RW