Add child_status_callback() function and arrange to call periodically.

At least on OS X 10.7, a call to apr_proc_wait(APR_NOWAIT) in fact seems to
block the caller. So instead of polling apr_proc_wait(), use APR callback
mechanism (apr_proc_other_child_register() et al.) and poll that using
apr_proc_other_child_refresh_all().
Evidently this polls the underlying system waitpid(), but the internal call
seems to better support nonblocking. On arrival in the
child_status_callback(APR_OC_REASON_DEATH) call, though, apr_proc_wait()
produces ECHILD: the child process in question has already been reaped.
The OS-encoded wait() status does get passed to the callback, but then we have
to use OS-dependent macros to tease apart voluntary termination vs. killed by
signal... a bit of a hole in APR's abstraction layer.
Wrap ensure_equals() calls with a macro to explain which comparison failed.