Hello Naoki,

  FN> Are / and /usr separated partition? If so, try
    > # echo "critical_filesystems_local="/var /usr" >> /etc/rc.conf

They are separate partitions, but adding
critical_filesystems_local="/var /usr" to rc.conf and uncommenting
wpa_supplicant=yes did not seem to work.  As a temporary measure, I
have reverted to launching wpa_supplicant from rc.local.  Many thanks
for the suggestion.

- Andy Ball