Remove assumptions about the max # channels in ioctl's:

Remove assumptions about the max # channels in ioctl's:
o change ioctl's that pass channel lists in/out to handle variable-size
  arrays instead of a fixed (compile-time) value; we do this in a way
  that maintains binary compatibility
o change ifconfig so all channel list data structures are now allocated
  to hold MAXCHAN entries (1536); this, for example, allows the kernel
  to return > IEEE80211_CHAN_MAX entries for calls like IEEE80211_IOC_DEVCAPS