nl80211: allow splitting wiphy information in dumps

The per-wiphy information is getting large, to the point
where with more than the typical number of channels it's
too large and overflows, and userspace can't get any of
the information at all.

To address this (in a way that doesn't require making all
messages bigger) allow userspace to specify that it can
deal with wiphy information split across multiple parts
of the dump, and if it can split up the data. This also
splits up each channel separately so an arbitrary number
of channels can be supported.

Additionally, since GET_WIPHY has the same problem, add
support for filtering the wiphy dump and get information
for a single wiphy only, this allows userspace apps to
use dump in this case to retrieve all data from a single
device.

As userspace needs to know if all this this is supported,
add a global nl80211 feature set and include a bit for
this behaviour in it.

Cc: Dennis H Jensen <dennis.h.jensen@siemens.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>