#8 pktmetas and 802.11

[Marco Canini]

at the moment the 802.11 mgmt frames are parsed and
rewritten even if no module is going to use them.
another approach would be to define a metadesc that
export some pktmetas which are pointers to the mgmt
header, and othe 802.11 stuff.
so if a module that needs mgmt frames is configured the
core will choose this richer metadesc and enable the
processing of mgmt frames. if no module requires it
then we don't waste memory and time to parse it.