Fix bug in handling AUTHENTICATE command (imap4d). Add user/group ACLs.

The bug was introduced in commit 6e2a3758.

The ACLs allow administrators to limit access to IMAP services to
particular users or users belonging to particular groups.

* imap4d/authenticate.c (imap4d_auth_capability): Take session
pointer as argument.  Don't report authentication capabilities
if tls is required.
(imap4d_authenticate): Fix conditional.
* imap4d/capability.c (imap4d_capability): Update call to
imap4d_auth_capability.
* imap4d/imap4d.c (cb_tls_required): Set tls_mode.
(auth_deny_user_list,auth_allow_user_list)
(auth_deny_group_list,auth_allow_group_list): New statics.
(check_user_groups,imap_check_group_list): New static functions.
(imap4d_cfg_param) <allow-users,deny-users>
<allow-groups,deny-groups>: New configuration statements.
(imap4d_session_setup0): Check allow/deny lists.
* imap4d/imap4d.h (imap4d_auth_capability): Change signature.

* pop3d/pop3d.c (cb_tls_required): Set tls_mode.

* bootstrap (gnulib_extra_files): Update.