L2CAP: Major refactor of the whole component.

(And surrounding portions of the "btCoreData" module.)

* Rewrote the main "l2cap.h" header representing protocol constants
and structures. Now conforms to general Haiku naming conventions
rather than BSD ones. Some more constants added/removed based
on the most recent Bluetooth specification.

* Rewrote all code derived from the BSDs to match Haiku conventions
and structures in the driver.

* Dropped the "channel" and "frame" structures from "btCoreData".
Channels are now managed by L2capEndpoints, and "frames" are
now just plain net_buffers without surrounding structures.
This also makes state management much simpler.

* Made it so that actual net_buffers are passed through to the
l2cap_receive function rather than another data structure.
A fake interface address is used to communicate connection
information. (This probably ought to be changed, though.)

* Get rid of l2cap_lower and l2cap_upper abstractions.
Everything related to channel/endpoint management is now
done in L2capEndpoint, while buffer reception is handled
directly in l2cap_receive and elsewhere, same as other drivers.

* Wire up more hooks and fix module flags (needed to be able to
get the module loaded and opening sockets at all.)

* Implement an actual locking strategy in L2capEndpoint
and HciConnection. There's still problems with lifetime
management, but at least thread-safety is mostly handled.

* Create an L2capEndpointManager and use it to manage
the endpoints, rather than having a single (unsafe)
linked-list.

And plenty of other refactorings and cleanups besides.
There's still more to be done for Bluetooth overall, though:

* The "btCoreData" and "hci" modules also badly need a major
overhaul, and should be merged into a single "bluetooth"
bus_manager. They also shouldn't be passing around pointers
to other modules like this.

* There's a number of TODOs/FIXMEs in the L2CAP module, most
notably around timeouts (especially command timeouts) and
parameter validation/specification.

Tested by myself with kallisti5's help. Incoming connections
(on the PSM for SDP) get all the way to the latter half
of the Configuration step before hanging.