Check for HCI_CHANNEL_MONITOR for Linux Bluetooth Monitor support.

Support the Linux Bluetooth Monitor mechanism if:

	we support Linux Bluetooth;

	struct sockaddr_hci includes an hci_channel field;

	HCI_CHANNEL_MONITOR is defined by the Bluetooth headers;

as there are some 3.x kernel versions in which struct sockaddr_hci
includes an hci_channel field but in which HCI_CHANNEL_MONITOR is not
defined.