ash: wait builtin should block

It was found that the 'wait' shell builtin would use 100% of a core
when waiting for a process to terminate:

   sleep 60 &
   wait

This is a regression caused by commit bb8f6b688 (ash: fix slow
running when background job is present).  See the commit message
and GitHub issue #434 for the long and involved history.

The problem is in the Windows implementation of waitproc() in ash.
The 'block' argument to waitproc() can take three values:

   DOWAIT_NONBLOCK
   DOWAIT_BLOCK
   DOWAIT_CHILD_OR_SIG

The first two have obvious meanings.  The third performs a non-
blocking wait(2) and if no PID is returned it waits for SIGCHLD.
So in effect it's a blocking wait.

The Windows implementation would perform a non-blocking wait(2)
but couldn't then wait for SIGCHLD, because Windows doesn't have
such a signal.  As a result the 'wait' builtin would loop calling
waitproc().

To avoid this DOWAIT_CHILD_OR_SIG should be treated as a blocking
wait in waitproc().

(GitHub issue #529)

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