Adding Solaris style extended attribute access for FreeBSD

[rmacklem]

Both FreeBSD and Linux currently use what I would call the "Linux style"
interface for extended attributes. However, given ZFS's heritage, the
underpinnings for a Solaris style interface is in the code.

I have now patched FreeBSD's main so it can do the "Solaris style" extended
attribute interface (a directory with the attribute names in it, plus the attributes
accessed like ordinary files) as an alternative to the "Linux style" (not replacing it).

I have a patch here:
https://reviews.freebsd.org/D49654
which adds the wiring to ZFS for this to work.
Except for defining a couple of new flag values used to inform zfs_zaccess()
that these are being used, all the changes are in
module/os/freebsd/zfs, with the biggest change being done to zfs_freebsd_lookup().

It currently seems to work ok once the "xattr" property is set to "dir".

Alexander Motin is aware of this and will look at it when he has time.

Do others have comments or suggestions w.r.t. this?