Destination address is at tx_buf[5] and tx_buf[6] rather than tx_buf[4] and
tx_buf[5].

Broadcast header is overridden by following code mistakenly.

XBee supports channels 0x0B-0x1A while XBee-PRO supports only 0x0C-0x17.

XBee sends short address even for `API_ID_TX_LONG_ADDR` if short address is
enabled. This results in check sum error of ICMPv6 since the IP address is
computed based on long address on the sender side while it is computed based on
short address on the receiver side.

`_set_addr` is called from `xbee_init` with lower bytes of the long address.
If `_set_addr` destructs the given address, the long address is also destructed.

When debugging multihop wireless network, it is useful to emulate non-transitive
network, that is, node A can communicate with B and B can communicate with C,
but A cannot communicate with C directly.

If `XBEE_DENIED_ADDRESSES`, which is an array of XBee long addresses, is
defined, packets from those addresses are dropped silently.

Example:
CFLAGS += "-DXBEE_DENIED_ADDRESSES={ 0x02, 0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0x02, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11 }"