Remove support for the GPG_AGENT_INFO envvar.

* agent/agent.h (opt): Remove field use_standard_socket.
* agent/command.c (cmd_killagent): Always allow killing.
* agent/gpg-agent.c (main): Turn --{no,}use-standard-socket and
--write-env-file into dummy options.  Always return true for
--use-standard-socket-p. Do not print the GPG_AGENT_INFO envvar
setting or set that envvar.
(create_socket_name): Simplify by removing non standard socket
support.
(check_for_running_agent): Ditto.
* common/asshelp.c (start_new_gpg_agent): Remove GPG_AGENT_INFO use.
* common/simple-pwquery.c (agent_open): Ditto.
* configure.ac (GPG_AGENT_INFO_NAME): Remove.
* g10/server.c (gpg_server): Do not print the AgentInfo comment.
* g13/server.c (g13_server): Ditto.
* sm/server.c (gpgsm_server): Ditto.
* tools/gpgconf.c (main): Simplify by removing non standard socket
support.
--

The indented fix to allow using a different socket than the one in the
gnupg home directory is to change Libassuan to check whether the
socket files exists as a regualr file with a special keyword to
redirect to another socket file name.

g13/server.c

@@ -612,15 +612,12 @@ g13_server (ctrl_t ctrl)
   if (opt.verbose || opt.debug)
     {
       char *tmp = NULL;
-      const char *s1 = getenv (GPG_AGENT_INFO_NAME);
 
       tmp = xtryasprintf ("Home: %s\n"
                           "Config: %s\n"
-                          "AgentInfo: %s\n"
                           "%s",
                           opt.homedir,
                           opt.config_filename,
-                          s1?s1:"[not set]",
                           hello);
       if (tmp)
         {