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

(cherry picked from commit 1b3748977f28c70e0b161fb476bf4e075bcc5940)