On startup, check that /dev/null exists and is a character device.
Otherwise, one of two things will happen: either /dev is a writable
directory and we will immediately create /dev/null as a regular file
and dump garbage into it, or it does not and we will spit out a stream
of error messages about failing to create /dev/null.
PR: 295782
MFC after: 1 week
Reviewed by: jhb, emaste
Differential Revision: https://reviews.freebsd.org/D57447
Dag-Erling Smørgrav
b5a96894f6