flowgrind: move parse_rpc_address to argparser
flowgrind-stop: allow to use IPv6 RPC adresses

This patch makes use of the parse_rpc_address function to
parse RPC addresses.

Closes #70 (flowgrind-stop does not accept IPv6 addresses)
Closes #159 (flowgrind-stop does not complain, if there is no address given)
flowgrind/flowgrind-stop: use asprintf instead of fixed size buffer
Merge pull request #160 from arnd/ipv6_stop

IPv6 stop