nl80211: explicit userspace MPM

Secure mesh had the implicit requirement that the Mesh
Peering Management entity be in userspace.  However
userspace might want to implement an open MPM as well, so
specify a mesh setup parameter to indicate this.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>