Separate process wait and close

On Linux, waitpid() both waits for the process to terminate and reaps it
(closes its handle). On Windows, these actions are separated into
WaitForSingleObject() and CloseHandle().

Expose these actions separately, so that it is possible to send a signal
to a process while waiting for its termination without race condition.

This allows to wait for server termination normally, but kill the
process without race condition if it is not terminated after some delay.