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>

ipapython/config.py

@@ -25,7 +25,6 @@ from optparse import (
 from copy import copy
 from configparser import SafeConfigParser
 from urllib.parse import urlsplit
-import socket
 import functools
 
 from dns.exception import DNSException
@@ -211,8 +210,11 @@ def __discover_config(discover_server = True):
                 servers = query_srv(name)
             except DNSException:
                 # try cycling on domain components of FQDN
+                # pylint: disable=ipa-forbidden-import
+                from ipalib.constants import FQDN
+                # pylint: enable=ipa-forbidden-import
                 try:
-                    domain = dns.name.from_text(socket.getfqdn())
+                    domain = dns.name.from_text(FQDN)
                 except DNSException:
                     return False