[kernel][heap] Don't use heap for thread stacks

Kernel stacks are now allocated by vm_allocate_kstack.

Thread creation no longer allows you to bring your own
stack. thread_create and thread_create_etc now always create a stack
of size DEFAULT_STACK_SIZE.

Remove THREAD_STACK_BOUNDS_CHECK feature as it has been subsumed by
guard pages provided by vm_allocate_kstack.

Test: verified no leaks using "k vmm kaspaces" and "k thread_tests"

ZX-2509 #done

Change-Id: I8dbde5c8a6852775b9796e0c7f963289291822b2

[zircon][syscalls] Make zx_time_t and zx_duration_t signed types

This change is part of a hard transition and breaks the layers above.

zx_time_t and zx_duration_t are now int64_t. ZX_TIME_INFINITE is now
INT64_MAX.

A negative time value passed to a syscall is treated as any other time
value that's "in the past".

Test: CQ; new unit tests; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I2a020cb2cf3dd7212608e24c1ee90cec58dd51ef

[kernel][cmd] Don't cast between function pointer types

Such casts are not valid in standard C++.

Change-Id: I99e5c95ee3c802bd9051904fffc3ac98aeec93d2

[kernel] Prefer fbl::count_of in C++ code

This also migrates some C++ code to use fbl::atomic, to help settle
some integer comparison differences.

Change-Id: Ief77949d571351a3990fe9a8251d56d35735422c

[kernel] Make spin_lock_held semantics match mutex

Prior to this change, spin_lock_held returned true when any CPU held
the lock. With this change it returns true only when the calling CPU
holds the lock.

Tested by running "k thread_tests" on x64 qemu+kvm and arm64 qemu with
1 and 4 CPUs.

ZX-2155 #done

Change-Id: I23c49394692c577013fa42e6156b75fe85a88ee0

[zircon] thread priority

This is the low level part. There is an API today
but it only operates on the current thread. The
change here allows it to operate on any thread_t

Tests added via 'k thread_tests'

Change-Id: I79beafd15634bb95c69c87846c139da6caa189b8

[kernel][wait_queue][test] add a test case to try to stress out the priority inherit mechanism

Change-Id: I5aa48d2a8b8783afb00b635abbb2674466b4cd4c

[kernel][tests] Fix printf() format string warnings

Enable -Wformat warnings for this directory by removing -Wno-format.

This involves:
* Size fixes for correctness/strictness.
* Signedness fixes, because we compile the kernel with
-Wformat-signedness (which GCC supports but Clang does not support).

Change-Id: Id1f3a73f4dbeee319e2aa80b05cc6648280dafd5

[kernel][thread] Remove unused "block" argument from thread_kill()

All the callers were passing block=false, so we can remove the
argument.

ZX-1490

Change-Id: I9c9c6ee7ecac0f1262638012c033dcb71bc6f12e

[kernel] thread_tests: Avoid passing block=true to thread_kill()

All the cases where the test calls thread_kill(t, true) are
immediately before thread_join(t,...). Passing block=true to
thread_kill() in that context is unnecessary because thread_join()
will also block in the same way: Both thread_kill() and thread_join()
do "wait_queue_block(&t->retcode_wait_queue,...)".

This is a step towards removing the "block" argument from
thread_kill().

ZX-1490

Change-Id: Idc9fce4e7ec2926872be9182049210efc0179c68

[kernel] Remove kernel/app module

This is no longer used at all.
The kernel/app/tests code doesn't use the "app" infrastructure.
So move it to kernel/tests.

Change-Id: I6cd8ec1bf569fd95940e5ebf4dc5f640b265eb69