dbus: Add global capabilities property

dbus: Add global capabilities property

Otherwise it's difficult to determine if the supplicant was built with
CONFIG_AP, CONFIG_IBSS_RSN, CONFIG_P2P, etc. CONFIG_AP and CONFIG_P2P
can be inferred from the introspection data of the Interface object,
but CONFIG_IBSS_RSN does not change the introspection data at all and
thus it's impossible to determine whether the supplicant supports it
without knowing its compile-time options.

Signed-hostap: Dan Williams <dcbw@redhat.com>
intended-for: hostap-1