tee: minor cleanup

Pull the open flags out of the loop into a local var. They won't be
changing, so this is marginally more readable.

Adds some extra brackets around the loop in preparation for a future
change that may try to fallback to opening the path as a socket if we
get an EOPNOTSUPP.

No functional change.

Reviewed by: asomers, des, emaste, ngie
Differential Revision: https://reviews.freebsd.org/D48196