Saves 128 bytes in testing with `mftest mega 1 -y`

And change verbiage to not refer to "Emergency Stop."

Co-Authored-By: Tim Moore <tim.moore@lightbend.com>
Co-Authored-By: AnHardt <github@kitelab.de>

- Send notifications to ExtUI for M0/M1

- wait_for_user can be non-volatile (not changed by interrupt)
  C / C++ compilers don't optimize away reads of non-volatile variables when a function call is used between accesses, because *any* variable could be changed by the function call. Since `wait_for_user` can't be changed without a function call, it should be non-volatile so the compiler can optimize away cases where it is read more than once without an intervening function call.

* Apply #pragma once in headers
* Adjust some thermistors formatting
* Misc cleanup and formatting

- Remove `print_job_timer.stop()` from `disable_all_heaters`
- Call `print_job_timer.stop()` for relevant `disable_all_heaters()`.
- Split up `kill()` for watchdog interrupt safety

Co-Authored-By: Zwaubel