Refine the way Ninja waits for subprocess completion.

This CL refactors how the Posix SubprocessSet implementation
delays with subprocess completion. Before this CL, the
Fuchsia version of Ninja would always wait for the output
pipe of a command to be closed to determine that the
corresponding "work" has completed.

This is important because a command sub-process can launch
child programs in the background and passing them their
stdout/stderr descriptors, and may exit *before* the child
itself.

However, this doesn't work in certain cases, for example if
the child process daemonizes itself, and keeps the output
descriptor opened, without writing anything to it. It is
suspected that this is the root cause of the Ninja
build timeouts that were are experiencing in CI (see
associated bug).

As a work-around, this CL changes the logic to used by
Ninja to the following:

- Each Subprocess instance can be in one of three
  states now:

  * Running: its PID is active, and its pipe is open.

  * Reaped: the PID has exited, but the pipe is still
    open and Ninja is waiting for more data out of it.

  * Finished: the PID has exited, and the pipe is
    closed.

  NOTE: The special case where the Subprocess has no
  pipe is no longer enabled in the Fuchsia version of
  Ninja, but corresponds to the upstream behavior
  for "console" processes. This didn't allow recording
  their output though.

- When the pipe is closed, the Subprocess is reaped
  and finished immediately.

- SIGCHLD is used to detect when any Subprocess PID exits
  and reaps it immediately (before that it would only
  check for upstream console processes). However, if
  the pipe is still open, a timer is setup to wait up
  to 30 seconds for incoming data on the pipe, and
  the Subprocess is kept in the running_ queue
  (but with 'reaped_ == true').

- When data arrives on the pipe of a reaped process,
  the timer is re-armed to wait for 30 more seconds
  (just in case).

- Otherwise, if the timer expires, the Subprocess
  is force-finished.

- A SIGINT / SIGTERM / SIGHUP should also force-finish
  reaped Subprocess instances immediately.

+ Adjust the process tree to print "REAPED [PID <pid>]"
  instead of "[PID <pid>]" when printing the command
  corresponding to a Reaped Subprocess instance.

+ Add a regression test that verifies that SIGINT
  stops Ninja immediately when it is waiting for
  a reaped subprocess.

+ Add a regression test that verifies that SIGTERM
  stops Ninja immediately when it is waiting for
  a reaped subprocess, and that the diagnostic
  message lists its pid with the REAPED tag.

Bug: 498320348
Fuchsia-Only: graceful-shutdown
Change-Id: Ic7e81aca2680fdc603873227be7b7515d8969248