4bitle: return type of Send.cancel should be error_t

was uint8_t but should have been error_t to match the
interface of AMSend.cancel.
Expose the LowPowerListening interface in ActiveMessageC for platforms
where this was not yet done. The remaining platforms where this is not
exposed are: eyesIFX (Tda5250, LowPowerListening is not implemented) and
tinynode (XE1205, has special LPL component and interface).
Remove confusing default channel define from RFxxx register definitions (from Elena Chervakova)
Escape serial CRC, allow limited retries.
Merge pull request #169 from carlsondc/dc

Escape serial CRC, allow limited retries.
CC2420X driver fixes

pull request 173: fixed a critical race condition that caused the driver
to hang under high loads, restructured code to eliminate long atomic
sections