[th/cleanup-2] some minor additions to th/cleanup and rework `process_commands()` to use cleanup

This is continuation to !1160 (merged) and the new cleanup mechanism.

Use cleanup for reaping services started via context.pexpect_service(). The purpose is to reuse the same mechanism, but more importantly, be in control of the order in which things get reaped. Per-step actions (context.pexpect_spawn()) still work the same and are reaped during process_commands().

Some reworking of context.pexpect_{spawn,service}(), like

• explicitly list the arguments
• add a "label" parameter

This conflicts with !1164 (merged), which should be merged first. Hence, this is still a draft.