WNM: Additional BSS Transition Management capability

WNM: Additional BSS Transition Management capability

Add some more functionality for BSS Transition Management:
- advertise support for BSS Transition Management in extended
  capabilities element
- add hostapd.conf parameter bss_transition=1 for enabling support
  for BSS Transition Management
- add "hostapd_cli disassoc_imminent <STA> <num TBTTs>" for sending
  disassociation imminent notifications for testing purposes
- wpa_supplicant: trigger a new scan to find another BSS if the
  current AP indicates disassociation imminent (TODO: the old AP needs
  to be marked to use lower priority to avoid re-selecting it)

Signed-hostap: Jouni Malinen <j@w1.fi>