agent: Add command "getinfo std_env_names".
* agent/command.c (cmd_getinfo): Add new sub-command.
The current output is:
> getinfo std_env_names D GPG_TTY D TERM D DISPLAY D XAUTHORITY D XMODIFIERS D GTK_IM_MODULE D QT_IM_MODULE D PINENTRY_USER_DATA OK
Note that there is an invisible \x00 at the end of each line.