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/debug.c

@@ -281,6 +281,8 @@ const char *DeviceExtensionKindSym(UINT32 Kind)
     {
     case FspFsctlDeviceExtensionKind:
         return "Ctl";
+    case FspFsmupDeviceExtensionKind:
+        return "Mup";
     case FspFsvrtDeviceExtensionKind:
         return "Vrt";
     case FspFsvolDeviceExtensionKind: