wpa_supplicant: new DBus API implementation

wpa_supplicant: new DBus API implementation

This patch implements the new DBus API. Both, the new and the
previous API may work concurrently and may be turned on or off
separately in .config file.
Some features of the new API are:
 - more wpa_supplicant's events are signaled with DBus signals,
 - introspection data (requires libxml2 and may be disabled),
 - CurrentBSS and CurrentNetwork properties,
 - PropertyChanged signal for most of properties,
 - Relatively easy to extend.
.config options for the new API are: CONFIG_CTRL_IFACE_DBUS_NEW=y and
CONFIG_CTRL_IFACE_DBUS_INTRO=y for introspection.

This commit misses couple of parts from the full implementation
(these are still under review):
- fetching all configuration parameters for learning WPS information
- scan result BSS add/remove notification (register_bss() and
  unregister_bss() notification callbacks)