This is a result of the second review.

Also, moved lease unit tests to a dedicated test file.

Also, allow the server to generate FQDN if configured to do so, even if
client specified non-empty FQDN.

This gives us a very tiny speedup, as size() was a const method already.

 - Dhcpv{4,6}Srv::classifyPacket() is now better commented.
 - constants are now used instead of magic numbers
 - Pkt{4,6}::addClass is now better commented

 - config parsers updated
 - new unit-tests for vendor options written
 - libdhcp++ cleaned up
 - fixes in option_vendor.cc
 - comments added and cleaned up

 - exceptions are now more verbose

 - comments updated
 - whitespaces cleaned up
 - minor refactoring in DHCPv6 Srv

Also added spaces around some operators.