sys: fsmup device

- This commit introduces the fsmup device, which is a major change in how
network file systems are handled. Previously every network file system's
fsvol device was directly registered with the MUP. Now there is a single
fsmup device that is registered with the MUP; network file systems' fsvol
devices register with fsmup instead. The fsmup device maintains a prefix
table which it uses to demultiplex and forward requests to the appropriate
fsvol device.
- This device change was necessatitated to fix issue #87.

src/sys/create.c

@@ -164,6 +164,8 @@ static NTSTATUS FspFsvolCreateNoLock(
 #pragma prefast(disable:28175, "We are a filesystem: ok to access Vpb")
             FileObject->Vpb = FsvolDeviceExtension->FsvrtDeviceObject->Vpb;
 
+        FileObject->FsContext2 = FsvolDeviceObject;
+
         Irp->IoStatus.Information = FILE_OPENED;
         return STATUS_SUCCESS;
     }