common: Remove code duplication for service starting.
* common/homedir.c (gpg_agent_socket_name): New. * common/asshelp.c (start_new_service): New. Based on start_new_gpg_agent. (start_new_gpg_agent): Divert to start_new_service. (start_new_dirmngr): Ditto.
This prepares for adding yet another service starting function.
- Signed-off-by: Werner Koch <wk@gnupg.org>