Bluetooth: Introduce trusted flag for management control sockets

Providing a global trusted flag for management control sockets provides
an easy way for identifying sockets and imposing restriction on it. For
now all management sockets are trusted since they require CAP_NET_ADMIN.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>