Move pressure-vessel-adverb subprocess supervision into common code

Continuation of !671 (merged) and !672 (merged), could be useful for steamrt/tasks#387.

• !671 (merged)

• !672 (merged)

• pv-adverb: Move low-level subprocess management into common code

  There's no reason why this particularly needs to be pressure-vessel-specific, and since commit 7627197e "steam-runtime-tools: Separate out modules not in the shared library" we can share code between pressure-vessel/ and bin/ without enlarging the shared library.

• pv-adverb: Factor out more subprocess management as SrtProcessManager

  No functional change intended other than a few debug messages, but this will let us recycle this code for a new steam-runtime-supervisor which does not depend on pressure-vessel-specific code and can easily be included in the scout runtime.

  Helps: steamrt/tasks#387