ntp: take auto_offline sources offline before sending new request
nameserv: check that address returned from gethostbyname() is IPv4

Also, always return failure with -6.
nameserv: add support for returning multiple addresses
ntp: try adding other server addresses

When adding a server from configuration file, don't give up when the
first returned address was already added for another server directive,
but try adding other addresses until one succeeds.
ntp: allow changing address of core instance
ntp: support pools

The pool directive can be used to configure chronyd for a pool of NTP
servers (e.g. pool.ntp.org). The name is expected to resolve to multiple
addresses which change over time.

On start, a source will be added for each resolved address. When a
source from the pool is unreachable or marked as falseticker, chronyd
will try to replace the source with a newly resolved address of the
pool.

The minimum interval between replacements is currently set to 244
seconds to avoid frequent DNS requests.