healthcheck: add possibility to check labels to select loopback IP
healthcheck: add possibility to check labels to select loopback IP

This allows one to use several instances of the healthchecker, each
instance handling a subset of the loopback IP. This is compatible only
with Linux currently. The documentation at the top has been updated for
that.

earies/exabgp,dneiter/exabgp,blablacar/exabgp,dneiter/exabgp,lochiiconnectivity/exabgp,benagricola/exabgp,fugitifduck/exabgp,earies/exabgp,PowerDNS/exabgp,dneiter/exabgp,lochiiconnectivity/exabgp,PowerDNS/exabgp,blablacar/exabgp,benagricola/exabgp,lochiiconnectivity/exabgp,fugitifduck/exabgp,blablacar/exabgp,earies/exabgp,fugitifduck/exabgp,PowerDNS/exabgp,benagricola/exabgp