BUG: childFDs mapping in process support breaks process termination

[exarkun]

Attached program demonstrates how it is impossible to shutdown certain processes
when using the childFDs mapping.
PP.processEnded tries to close its file descriptors in two different ways -
either with os.close() or transport.closeChildFD().  Only the os.close() path works.
I'm not sure if Twisted really should be responsible for knowing how to close
these FDs, though, since they were created outside of Twisted.  At the very
least, though, it is rather unfortunate that closeChildFD silently ignores the
unknown fds it is passed, making this problem much more difficult to debug than
it needed to be.