agent: New option --no-user-trustlist and --sys-trustlist-name.

* agent/gpg-agent.c (oNoUserTrustlist,oSysTrustlistName): New.
(opts): Add new option names.
(parse_rereadable_options): Parse options.
(finalize_rereadable_options): Reset allow-mark-trusted for the new
option.
* agent/agent.h (opt): Add fields no_user_trustlist and
sys_trustlist_name.
* agent/trustlist.c (make_sys_trustlist_name): New.
(read_one_trustfile): Use here.
(read_trustfiles): Use here.  Implement --no-user-trustlist.  Also
repalce "allow_include" by "systrust" and adjust callers.
--

With the global options we can now avoid that a user changes the
Root-CA trust by editing the trustlist.txt.  However, to implement
this we need a new option so that we don't need to rely on some magic
like --no-allow-mark-trusted has been put into a force section.

The second option makes system administration easier as it allows to
keep the trustlist in a non-distributed file.

GnuPG-bug-id: 5990
Backported-from-master: 1530d04725

agent/agent.h

@@ -148,6 +148,13 @@ struct
      interactively mark certificate in trustlist.txt as trusted. */
   int allow_mark_trusted;
 
+  /* Only use the system trustlist.  */
+  int no_user_trustlist;
+
+  /* The standard system trustlist is SYSCONFDIR/trustlist.txt.  This
+   * option can be used to change the name.  */
+  const char *sys_trustlist_name;
+
   /* If this global option is true, the Assuan command
      PRESET_PASSPHRASE is allowed.  */
   int allow_preset_passphrase;