Disable Rock store and shared memory cache if shared memory is not available.

Disable Rock store and shared memory cache if shared memory is not available.

The patch adds support for building on systems without POSIX shared
memory (e.g. OpenBSD).  IpcIo DiskIO module and Rock store is not
enabled by default if shared memory support is not available.  If IpcIo
or Rock store is explicitly enabled, but no shared memory support is
available, fail with error during configure.  Fail with error at
runtime if shared memory cache is enabled but no shared memory support
is detected.