Add SIGINT protection to our `pdbpp` based debugger subystem such that
for (single-depth) actor trees in debug mode we ignore interrupts in any
actor currently holding the TTY lock thus avoiding clobbering IPC
connections and/or task and process state when working in the REPL.

As a big note currently so called "nested" actor trees (trees with
actors having more then one parent/ancestor) are not fully supported
since we don't yet have a mechanism to relay the debug mode knowledge
"up" the actor tree (for eg. when handling a crash in a leaf actor).
As such currently there is a set of tests and known scenarios which will
result in process cloberring by the zombie repaing machinery and these
have been documented in https://github.com/goodboy/tractor/issues/320.

The implementation details include:

- utilizing a custom SIGINT handler which we apply whenever an actor's
  runtime enters the debug machinery, which we also make sure the
  stdlib's `pdb` configuration doesn't override (which it does by
  default without special instance config).
- litter the runtime with `maybe_wait_for_debugger()` mostly in spots
  where the root actor should block before doing embedded nursery
  teardown ops which both cancel potential-children-in-deubg as well
  as eventually trigger zombie reaping machinery.
- hardening of the TTY locking semantics/API both in terms of IPC
  terminations and cancellation and lock release determinism from
  sync debugger instance methods.
- factoring of locking infrastructure into a new `._debug.Lock` global
  which encapsulates all details of the ``trio`` sync primitives and
  task/actor uid management and tracking.

We also add `ctrl-c` cases throughout the test suite though these are
disabled for py3.9 (`pdbpp` UX differences that don't seem worth
compensating for, especially since this will be our last 3.9 supported
release) and there are a slew of marked cases that aren't expected to
work in CI more generally (as mentioned in the "nested" tree note
above) despite seemingly working  when run manually on linux.