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

(cherry picked from commit 5690261858b6bd8f7d09eda2ae74f3def2d69a01)