Recognize "ro", "rdonly", "norw", "rw" and "noro" as equal options in
vfs_equalopts(). This allows vfs_sanitizeopts() to filter redundant
occurrences of these options. It was possible that for example both "ro"
and "rw" options became active concurrently.

PR: kern/133614
Discussed on: freebsd-hackers
MFC after: 1 month