core: don't unset reload result, unless we begin a start or reload operation

core: don't unset reload result, unless we begin a start or reload operation

Much like the result of the service itself we should not reset the
reload result unless we actually start from the beginning, so that
clients can query it at any time.

Specifically, let's reset the result states only when we begin with a
start operation (for both the main result, and the reload result), when
we begin with a reload operation (only for the load result), or when the
use explicitly asks for that via "systemctl reset-failed".

This is a more generic fix for #1447.

Fixes #1447.