Bug 5254, part 1: Do not leak master process' cache.log to kids (#1222)

Bug 5254, part 1: Do not leak master process' cache.log to kids (#1222)

The fork()ed kids unknowingly inherited cache.log file descriptor and,
hence, prevented the underlying file from being deleted on log rotation.

This change does not fully fix the bug because the file is still being
held open by the master process itself. This change was isolated because
it lacks bad/controversial side effects -- it is a simple step forward.

This fix may also help stop leaking kid's cache.log descriptor to
helpers, but that requires more work -- replacing descriptor duping
trick in ipcCreate() with a proper servicing of helper stderr descriptor
via Comm. For now, each helper process still keeps cache.log alive.