better error handling on server startup
better error handling on server startup

- detect if stale corectld.store or corectld.nameserver somehow
  were left running from previous sessions and if found kill them
  and continue initialization instead of just popping an error and
  exiting
- exit with a much more conscise error message if corectld.store or
  corectld.nameserver ports are found binded to some foreign app

Signed-off-by: António Meireles <9056f7c1f560fe4e0c75ab3cde8bee0cf3d504f4@reformi.st>

AntonioMeireles/coreos-xhyve,TheNewNormal/corectl,AntonioMeireles/coreos-xhyve,TheNewNormal/corectl