pmount: fixed automounting drives in PUPMODE=13

_pmount_ adds line(s) like:
```mount -t ext4 /dev/sda2 /mnt/sda2 #pmount```
to /etc/rc.d/rc.local, however in PUPMODE=13, where snapmerge excludes the whole /mnt dir from being merged onto pup_ro1, appropriate subdirs (sdaX, sdbX, etc.) don't exist when system starts up.
Simple 'mkdir -p' did the job.

Nice feature, btw. Just discovered it right now - I was using a separate app for this before. So many years and still surprises! :)

Greetings!