journalctl: give a nice hint about group membership based on ACLs of /var/log/journal

If we notice that we unprivileged and not in any of the groups which
have access to /var/log/journal, print a nice message about which groups
do.

This checks and prints all groups that are in the default ACL for
/var/log/journal, which is not necessarily correct for all journal
files, but pretty close.
util: be more picky when validating hostnames

No longer allow dots at the beginning or end of host names, Or double
dots.

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1152187/comments/14
hostnamed: pretty_string_is_safe() already exists in string_has_cc(), so use that