Investigate dhcp{4,6} start-up merge possiblity

[tomek]

DHCPv4 and DHCPv6 servers have many similarities:

• main() functions in src/bin/dhcp{4,6}/main.cc
• ControlledDhcpv?{4,6}Srv classes.
• TestDhcpv6Daemon from srv/bin/dhcp{4,6}/tests/dhcp{4,6}_test.py

Some form of merge should be investigated. A common base class, or perhaps template would be helpful.

It should be noted that in some cases the code is similar now, but is expected to grow differently in the future, so unification should not be the automatic answer and some thought should be given.