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>

src/common/ieee802_11_defs.h

@@ -538,6 +538,16 @@ struct ieee80211_mgmt {
 					 * Entries */
 					u8 variable[0];
 				} STRUCT_PACKED bss_tm_req;
+				struct {
+					u8 action; /* 8 */
+					u8 dialog_token;
+					u8 status_code;
+					u8 bss_termination_delay;
+					/* Target BSSID (optional),
+					 * BSS Transition Candidate List
+					 * Entries (optional) */
+					u8 variable[0];
+				} STRUCT_PACKED bss_tm_resp;
 			} u;
 		} STRUCT_PACKED action;
 	} u;