mac80211: Let userspace enable and configure vendor specific path selection.

Userspace will now be allowed to toggle between the default path
selection algorithm (HWMP, implemented in the kernel), and a vendor
specific alternative.  Also in the same patch, allow userspace to add
information elements to mesh beacons.  This is accordance with the
Extensible Path Selection Framework specified in version 7.0 of the
802.11s draft.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>