Add a new "untrusted" option to the mount command. Its purpose

Add a new "untrusted" option to the mount command. Its purpose
is to notify the kernel that the file system is untrusted and it
should use more extensive checks on the file-system's metadata
before using it. This option is intended to be used when mounting
file systems from untrusted media such as USB memory sticks or other
externally-provided media.

It will initially be used by the UFS/FFS file system, but should
likely be expanded to be used by other file systems that may appear
on external media like msdosfs, exfat, and ext2fs.

Reviewed by:  kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20786