Bluetooth: Add generic mgmt helper API

There are several mgmt protocol features that will be needed by more
than just the current HCI_CHANNEL_CONTROL. These include sending generic
events as well as handling pending commands. This patch moves these
functions out from mgmt.c to a new mgmt_util.c file.

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