rc.d/linux: Attempt to mount only if necessary

Currently, if the linux service is run twice, mount(8) fails with:

mount: linprocfs: Device busy
mount: linsysfs: Device busy
mount: devfs: Device busy
mount: fdescfs: Device busy
mount: tmpfs: Device busy

It is a bit more user-friendly if before running mount(8) the service
checks if there are any file systems left to be mounted. This patch
implements this behavior.

Also, while here, create mount points directories (as suggested by
otis).

Reviewed by: trasz
Approved by: trasz (src)
Differential Revision: https://reviews.freebsd.org/D32463