introduce new IEEE80211_STA_ONLY kernel option that can be set to

introduce new IEEE80211_STA_ONLY kernel option that can be set to
remove IBSS and HostAP support from net80211 and 802.11 drivers.
it can be used to shrink RAMDISK kernels for instance (like what
was done for wi(4)).
it also has the benefit of highlighting what is specific to IBSS
and HostAP modes in the code.
the cost is that we now have two code paths to maintain.