shm: Port shared memory plugin to Windows

Submitted by Joshua M. Doe

Link to original bug (#698657)

Description

I've ported shmsink and shmsrc to Windows. I've used the simplest method possible, just #ifdef'ing code to swap out Unix sockets for TCP sockets, and to use Windows shared memory functions CreateFileMapping/MapViewOfFile instead of shm_open/mmap. Permissions are much trickier on Windows, so I've opted to #ifdef that property out. I'll attach a patch as soon as I get release approval, but before then I'd like to know if my method is acceptable.