service: attempt to execute next main command only for oneshot services (#6619)

service: attempt to execute next main command only for oneshot services (#6619)

This commit fixes crash described in
https://github.com/systemd/systemd/issues/6533

Multiple ExecStart lines are allowed only for oneshot services
anyway so it doesn't make sense to call service_run_next_main() with
services of type other than SERVICE_ONESHOT.

Referring back to reproducer from the issue, previously we didn't observe
this problem because s->main_command was reset after daemon-reload hence
we never reached the assert statement in service_run_next_main().

Fixes #6533