core: when determining whether a process exit status is clean, consider whether it...

core: when determining whether a process exit status is clean, consider whether it is a command or a daemon

SIGTERM should be considered a clean exit code for daemons (i.e. long-running
processes, as a daemon without SIGTERM handler may be shut down without issues
via SIGTERM still) while it should not be considered a clean exit code for
commands (i.e. short-running processes).

Let's add two different clean checking modes for this, and use the right one at
the appropriate places.

Fixes: #4275