Async signals support in Non-SFI mode
=====================================

Issue: https://code.google.com/p/nativeclient/issues/detail?id=4065

This provides a way to asynchronously interrupt another thread in the same
process, in a similar fashion to POSIX signals.  Signal support is limited to
Non-SFI mode (see nacl_irt_async_signal_handling in src/untrusted/irt/irt.h).

Async signals have several differences from POSIX signals:

* Synchronous signals (from hardware exceptions) are separate and their behavior
  is not changed.  Furthermore, synchronous signals cannot be handled with this
  interface.
* There is a single type of signal, and only a single, global async signal
  handler is supported.  This means that there is no support for POSIX signal
  numbers.
* There is no way to block signals, not even when the signal handler is running.
* There is no equivalent to sigaltstack(), so the signal handler always runs on
  the same stack as the thread.
* We don't provide libc wrapper functions for this interface in libnacl at the
  moment.  If full POSIX support is needed, it can be implemented in user code,
  on top of the IRT interfaces provided.
* NaCl signals are not intended to abort any in-process operations (such as
  syscalls, IRT calls or PPAPI calls) and they will restart once the signal
  handler returns.  There are two exceptions that will be interrupted and do
  fail with EINTR:
  * futex_wait_abs() with a non-NULL timeout
  * nanosleep()

Similar to POSIX signals, NaCl signals are delivered the next time the thread is
scheduled to run but before giving the process control of execution.  This also
means that if several signals are sent to a thread before it is scheduled to
run, a single async signal will be delivered, and the signal handler will be run
just once.  That also means that the signal handler can run at any point during
the program execution, so the signal handler must be written with care to avoid
doing unsafe operations, such as acquiring mutexes that may be held by the
interrupted thread.  Invoking any unsafe operation from within a signal handler
is undefined.  NaCl only guarantees that the following IRT interfaces are
async-signal-safe and can be called from within the signal handler:

* tls_get()
* futex_wait_abs()
* futex_wake()
* send_async_signal()

In order to deliver signals to the correct thread, a new version of the
nacl_irt_thread IRT functions has been introduced which will assign an opaque
thread identifier to each new thread and will populate it into the |child_tid|
parameter.  Since the initial thread is not created using nacl_irt_thread, the
constant NACL_IRT_MAIN_THREAD_TID can be used to refer to it.  This thread
identifier can be used as a parameter to send_async_signal().  Providing an
invalid thread identifier that is not NACL_IRT_MAIN_THREAD_TID, was not obtained
from thread_create, or refers to a thread that has already exited may cause
undefined behavior.  For example, this could include sending the signal to an
internally-created thread, if the thread ID is reused for a new thread.