POSIX Shared memory, revised, again

So I've finished reformulating this patch to use the device/inode
instead of the hash idea I had earlier.

I have tested this patch (on Darwin) to permit the postmaster to
recover after a crash or killing, and it will not permit the
postmaster to restart while another postmaster or backends are still
in the database (regardless of the lockfile status). The ideas are
the same as before, but are described via flowchart in the attached
PDF. The three template files opt-out those particular templates from
the POSIX shmem, but this could be done alternately by opting-in the
desired patches. An autoconf'ing of the patched configure.in file is,
of course, required.

I appreciate the feedback and consideration.

Thanks again,
Chris Marcellino