core: consider non-SERVICE_EXEC_START commands for EXIT_CLEAN_COMMAND

core: consider non-SERVICE_EXEC_START commands for EXIT_CLEAN_COMMAND

When there are multiple ExecStop= statements, the next command would continue
to run even after TimeoutStopSec= is up and sends SIGTERM. This is because,
unless Type= is oneshot, the exit code/status would evaluate to SERVICE_SUCCESS
in service_sigchld_event()'s call to is_clean_exit(). This success indicates
following commands would continue running until the end of the list
is reached, or another timeout is hit and SIGKILL is sent.

Since long running processes should not be invoked in non-SERVICE_EXEC_START
commands, consider them for EXIT_CLEAN_COMMAND instead of EXIT_CLEAN_DAEMON.
Passing EXIT_CLEAN_COMMAND to is_clean_exit() evaluates the SIGTERM exit
code/status to failure and will stop execution after the first timeout is hit.

Fixes #11431