Add an option to create interface of a certain type with INTERFACE_ADD

Add an option to create interface of a certain type with INTERFACE_ADD

Some drivers do not support having two station interfaces, so the fact
that wpa_supplicant always creates a new interface in station mode, even
if it will be used as another type of interface, may prevent
wpa_supplicant from creating new interfaces. Allow setting the interface
type when a new interface is created so that interfaces of supported
types can be created.

Currently supported types are station ("sta") and AP ("ap"). If the
interface type is not specified, a station interface will be created.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>