S9 EXT  (sys:wait)             ==>  list
        (sys:waitpid integer)  ==>  integer | #f

Wait for a child process to terminate and return the exit information
and of the terminated process. SYS:WAIT returns a list containing the
exit code and PID of any exited child process:

        (exit-code PID)

When no exit information could be obtained it returns #F. The SYS:WAITPID
procedure checks whether a process with a specific PID of INTEGER has
exited and if so, it returns its exit code. When the process exists but
has not yet exited, it returns #F. When there are no child processes to
wait for, it signals an error. SYS:WAITPID, unlike SYS:WAIT, does not
block the calling process.
