Bluetooth: Add new BT_SNDMTU and BT_RCVMTU socket options

This patch adds new socket options for LE sockets since the existing
L2CAP_OPTIONS socket option is not usable for LE. For now, the new
socket options also require LE CoC support to be explicitly enabled to
leave some playroom in case something needs to be changed in a backwards
incompatible way.

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