lib: Expose type-specific systemd unit details
lib: Expose type-specific systemd unit details

This makes the proxies useful for other kinds, such as timers.

Closes  #12728
moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,garrett/cockpit