Add knob to limit hostname length
    
    On Linux systems the length limit for hostnames is hardcoded
    at 64 in MAXHOSTNAMELEN
    
    Solaris, for example, allows 255 characters, and DNS allows the
    total length to be up to 255 (with each label < 64).
    
    Add a knob to allow configuring the maximum hostname length (FQDN)
    
    The same validators are used between hosts and DNS to apply
    the knob only when dealing with a FQDN as a hostname.
    
    The maxlen option is included so installers can limit the length
    of allowed hostnames when the --hostname option is used.
    
    https://pagure.io/freeipa/issue/2018
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>