cleanups in Action::State

cleanups in Action::State

- Rename "ready" to "inited", for symmetry to s4u::Activity
- Remove the unused state "to_free"
- Rename the funky state "not_in_the_system" into "ignored", and
  improve its documentation.

Further cleanups could include not having any action set beyond the
ones existing in kernel::resource::Model, so that
kernel::resource::Action::get_state() see them all.

CpuCas01 is the one heavily using ignored actions, as failure detectors.