O_PATH: Allow to open symlink

When O_NOFOLLOW is specified, namei() returns the symlink itself. In
this case, open(O_PATH) should be allowed, to denote the location of symlink
itself.

Prevent O_EXEC in this case, execve(2) code is not ready to try to execute
symlinks.

Reported by: wulf
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29323