SpinelNcpInstance: Add support for newly added NCP counters.
SpinelNcpInstance: Add support for newly added NCP counters.

This commit adds support to wpantund for newly added NCP counters
which track number of events/messages through the NCP (e.g., number
of IP packets sent/received (secure vs insecure), number of dropped
IP packets, number of spinel frames, etc.

aeliot/wpantund,d-bahr/wpantund,aeliot/wpantund,aeliot/wpantund,openthread/wpantund,abtink/wpantund,d-bahr/wpantund,openthread/wpantund,abtink/wpantund,aeliot/wpantund,d-bahr/wpantund,openthread/wpantund,openthread/wpantund,d-bahr/wpantund,abtink/wpantund,abtink/wpantund