Remove buffer size limit for ports

[axeld]

The current port implementation has an upper limit for the size of the transferred message (PORT_MAX_MESSAGE_SIZE) of 64kB. The BeOS documentation clearly states that there is no such limit, so it has to be removed (or at least substantially lifted).

In BeOS, the limit seems to be around 2 MB per message, and a total of 8 MB for all ports at once.

Additionally, you could also implement things like shared memory to save allocating and copying big chunks of memory in the kernel for large messages when possible.

[axeld]

The port message size is now limited to 256k, and there is a global limit of 64 MB of messages in all ports in the system.

Having a limit makes sense, since this is a restricted shared resource. BMessages already use areas for larger messages.