tee: try opening as a UNIX socket if open(2) fails

If we get EOPNOTSUPP from open() failing, then it may just be that we're
looking at a unix(4) socket instead of a normal file/device or whatnot.
Fallback to trying to open it as a unix(4) socket, which is a useful
feature that doesn't add much complexity.

Reviewed by: des, emaste, markj
Differential Revision: https://reviews.freebsd.org/D48197