agent: Add DBUS_SESSION_BUS_ADDRESS et al. to the startup list.

* agent/gpg-agent.c (agent_copy_startup_env): Replace explicit list
with the standard list.

Although the function agent_copy_startup_env is newer than
session_env_list_stdenvnames the latter was not used. When
DBUS_SESSION_BUS_ADDRESS was added to the latter it was forgotten to
add it to the former as well. Having all stdnames here seems to be
the Right Thing (tm) to do.

• GnuPG-bug-id: T3947
• Signed-off-by: Werner Koch <wk@gnupg.org>