Improve socket activation support.

[alopukhov]

https://github.com/rootless-containers/rootlesskit/issues/428 adds handy ability to keep systemd socket activation sockets. Still there is room for improvement: as @charliemirabile mentioned fixing LISTEN_PID env variable would be nice.

As far as I can understand, the way rootlesskit works now

sequenceDiagram
    participant Parent
    participant Child
    participant Target
    Parent ->> Child: Spawns rootlesskit
    Child --> Child: (optionally) Reexec self for caps
    Child ->> Target: Spawns Target

I think it's possible to replace this flow with following one to correct LISTEN_PID:

sequenceDiagram
    participant Parent
    participant Child
    participant Activation helper
    Parent ->> Child: Spawns rootlesskit
    Child --> Child: (optionally) Reexec self for caps
    Child ->> Activation helper: Spawns rootlesskit
    Activation helper ->> Activation helper: Changes LISTEN_PID
    Note over Activation helper: Replaces self with Target using exec.

[AkihiroSuda]

Implemented in:

• 449