Currently all radios receive all traffic on the simulated air
if they are tuned to the same channel. This patch introduces
the concept of grouping, which allows to assign a radio to
certain group. Only radios in the same group can 'see' each other.

Each bit in /debug/ieee80211/phy*/hwsim/group
represents one group. By default all radios belong to the same group "1",
e.g. bit 1 is set. Additionally a radio can belong to several groups.
Signed-off-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>