Add support for mounting single files in nullfs

My main use-case for this is to support mounting config files and secrets
into OCI containers. My current workaround copies the files into the
container which is messy and risks secrets leaking into container images
if the cleanup fails.

Reviewed by: mjg, kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D37478

(cherry picked from commit a3f714c4ff8cf3754520f330abe783aa6a06dcdb)