Add two options to allow mount to avoid covering up existing mount points.
The two options are

• nocover/cover: Prevent/allow mounting over an existing root mountpoint.

E.g., "mount -t ufs -o nocover /dev/sd1a /usr/local" will fail if /usr/local
is already a mountpoint.

• emptydir/noemptydir: Prevent/allow mounting on a non-empty directory.

E.g., "mount -t ufs -o emptydir /dev/sd1a /usr" will fail.

Neither of these options is intended to be a default, for historical and
compatibility reasons.

Reviewed by: allanjude, kib
Differential Revision: https://reviews.freebsd.org/D21458