Win32 shmem

Attached is a first attempt at a "native win32 shmem implementatio",
based on previous discussions. Instead of emulating the sysv stuff. The
base stuff (using mapped pagefile) is still the same, of course.

Needs more testing, but has survived my tests so far. And unlike the
previous implementation, it does properly refuse to start a second
postmaster in a data directory where there is one or more backends still
running.

Does it seem like I've overlooked anything obvious in this? I do get the
feeling that this is too simple, but I don't know exactly where the
problem is :-)

//Magnus