`thread_measure_stack_free()` previously assumed that reading past the
stack is safe. When the stack was indeed part of a thread, the
`thread_t` structure is put after the stack, increasing the odds of
this assumption to hold. However, `thread_measure_stack_free()` could
also be used on the ISR stack, which may be allocated at the end of
SRAM.
A second parameter had to be added to indicate the stack size, so that
reading past the stack can now be prevented.
This also makes valgrind happy on `native`/`native64`.
Declare sched_active_thread and sched_active_pid locally in the ESP code for
now. Once the code is cleaned up to no longer tap into scheduler internals but
use the API instead, those can be dropped again.
