common: Prepare for more flags in start_new_service.
* common/asshelp.h (ASSHELP_FLAG_AUTOSTART): New. * common/asshelp.c (start_new_service): Rename arg autostart to flags and adjust checks. (start_new_gpg_agent): Likewise. Change all callers. (start_new_keyboxd): Likewise. Change all callers. (start_new_dirmngr): Likewise. Change all callers.
It is easier to have a generic flags arg instead of adding more and
more dedicated args. verbose and debug are kept as they are because
they are not boolean.