Add "fake" shared memory segment implementation.

Add "fake" shared memory segment implementation.

"Fake" shared memory segments are enabled #if a system does not
support POSIX shared memory.  Such segments use regular new to
allocate memory, it is not shared among multiple processes.  The
purpose of the change is to allow code that uses Ipc::Mem::Segment to
run in non-SMP mode (that is when only a single process, except for
master, is running) on systems without POSIX shared memory support.

Note: running SHM-using features in SMP mode still requires POSIX
shared memory support, the patch does not change that.