hostapd: Allow external management frame processing or testing

hostapd: Allow external management frame processing or testing

This enables more convenient protocol testing of station side
functionality in various error cases and unexpected sequences without
having to implement each test scenario within hostapd.
ext_mgmt_frame_handle parameter can be set to 1 to move all management
frame processing into an external program through control interface
events (MGMT-RX and MGMT-TX-STATUS) and command (MGMT_TX).

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