Unify access to FQDN
    
    FreeIPA's Python and C code used different approaches to get the FQDN of
    the host. Some places assumed that gethostname() returns a FQDN. Other
    code paths used glibc's resolver to resolve the current node name to a
    FQDN.
    
    Python code now uses the ipalib.constants.FQDN where a fully qualified
    domain name is expected. The variable is initialized only once and avoids
    potential DNS lookups.
    
    C code uses a new helper function ipa_gethostfqdn() in util package. The
    function implements similar logic as gethostfqdn() except it uses more
    modern getaddrinfo(). The result is cached as well.
    
    Fixes: https://pagure.io/freeipa/issue/8501
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>