Bluetooth: hci_sock: Add support for BT_{SND,RCV}BUF

This adds support for BT_{SND,RCV}BUF so userspace can set MTU based on
the channel usage.

Fixes: https://github.com/bluez/bluez/issues/201

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>