Add ctrl_iface command for triggering a roam to a specific BSS

Add ctrl_iface command for triggering a roam to a specific BSS

'wpa_cli roam <bssid>' can now be used to test roaming within an ESS
(e.g., for FT over-the-air). This command will bypass a new scan and
will select the BSS based on the specified BSSID. It is responsibility
of the caller to make sure that the target AP is in the BSS table.
This can be done, e.g., by running a scan before the roam command,
if needed.