check_ntp_peer: does not allow unix socket for "-H"

[waja]

In Debian we got the following bug reported:

the help text for the "-H" option states:

 -H, --hostname=ADDRESS
     Host name, IP Address, or unix socket (must be an absolute path)

but it uses "is_host()" to test for a valid ip address or dns-hostname.
This behavior has not changed between 2.2-3 (stretch) and 2.2-6 (sid).

It should either not say "unix socket" in the help text or allow the
usage of unix sockets.