Add a -h flag, and only attempt to recognize 802.11s mesh headers if it's set.

Add a -h flag, and only attempt to recognize 802.11s mesh headers if it's set.

I give up.  I have no access to the 802.11s drafts, I can't find
anything that suggests whether a heuristic check for an 802.11s header
should check for To DS and From DS both being set or either being set or
unset, or whether it should check for a QoS frame type (the examples in
all the documentation I can find have To DS and From DS set, and have a
QoS field, in the 802.11 header, but that might just be an example
802.11 header showing all the fields), so I'm just adding a -h
command-line flag; you need to specify it to get tcpdump to try to guess
whether a frame has a mesh header or not.  I'll leave it up to somebody
else to figure out what the best heuristic for detecting the presence of
mesh headers is (note that tcpdump and Wireshark have different
heuristics, both of which can probably get false positives, especially
with encrypted frames where the first payload byte just *happens* not to
have any of the reserved bits in the mesh header flags set).