rpc: add capture append flag to capture rpc msg.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabbad: open pcap in append mode when needed.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabba: add '--append' capture cli option.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
Merge pull request #7 from eroullit/append-pcap

Append packets to a pcap file