agent: fix for UPDATESTARTUPTTY

[NIIBE Yutaka]

In 2.0.20, I found that:

    $ gpg-connect-agent updatestartuptty /bye

doesn't work.

There are two problems for me.

(1) When I want to use pinentry for nterminal, there is no way,
    if I already have gpg-agent with DISPLAY.

(2) UPDATESTARTUPTTY doesn't work to switch TTY for pinentry for SSH.

For (1), I think that we need to implement something to negate X
Window System.

Latter is simply a bug.

Current implementation:

    In the function start_command_handler_ssh, the logic puts priority
    on ctrl->session_env which is initialized by
    agent_init_default_ctrl.  There are always GPG_TTY and TERM
    defined, because lines around 968 in gpg-agent.c, it says:

	/* Make sure that we have a default ttyname. */

    While UPDATESTARTUPTTY updates opt.startup_env, it doesn't affect
    at all.

Here is a patch to point the issue.  Tested and works for me.

diff --git a/agent/command-ssh.c b/agent/command-ssh.c
index 2f96ef5..6009956 100644
--- a/agent/command-ssh.c
+++ b/agent/command-ssh.c
@@ -3025,8 +3025,7 @@ start_command_handler_ssh (ctrl_t ctrl, gnupg_fd_t sock_client)
     const char *value;
 
     for (idx=0; !err && names[idx]; idx++)
-      if (!session_env_getenv (ctrl->session_env, names[idx])
-          && (value = session_env_getenv (opt.startup_env, names[idx])))
+      if ((value = session_env_getenv (opt.startup_env, names[idx])))
         err = session_env_setenv (ctrl->session_env, names[idx], value);
 
     if (!err && !ctrl->lc_ctype && opt.startup_lc_ctype)
--