Implemented file_cache_fallback mechanism

Implemented file_cache_fallback mechanism

which is essential as an attempt to fix the "failed to reattach"
error on Windows. If file_cache is enabled, Opcache will
automaticaly switch to file_cache_only mode in the case a process
failed to map the shared segment at the required address. The
important small part of the SHM will still be mapped, which
allows information exchange between normal processes using SHM
and those using the fallback mechanism.

This is based on Dmitry's, Matt's and mine ideas. So many thanks for
support!