Fix a "Kernel has child processes" warning even when it doesn't.
Michel Schrameck reported that the above warning was coming from a
very simple, obviously not fork()ing test program. I was able to
reproduce the problem with the test case when USE_SIGCHLD was turned
off. This fix changes the way first polling of SIGCHLD happens. It
passes all known tests, but it may negatively affect older programs.
Please report any problems.