Windows: allow cancelling long mount operations

Add a root-driver abort IOCTL that bypasses the mount control mutex and sets cooperative KDF abort flags for the active mount.

Restrict abort requests to privileged callers or to the user that initiated the pending mount, and retry early wait-dialog cancel requests until the driver has registered the cancellable mount context.

Wire the wait dialog Cancel button to send the abort request through a fresh driver handle, and propagate ERR_USER_ABORT through header/cache processing.

Add a /cancelmount command-line switch that sends the same abort request without displaying UI, so users can cancel hidden-wait-dialog mount operations from another process.

src/Driver/Ntdriver.h

@@ -26,6 +26,22 @@ typedef struct _THREAD_BLOCK_
 	MOUNT_STRUCT *mount;
 } THREAD_BLOCK, *PTHREAD_BLOCK;
 
+#ifndef SECURITY_MAX_SID_SIZE
+#define SECURITY_MAX_SID_SIZE 68
+#endif
+
+typedef struct _MOUNT_CANCEL_CONTEXT_
+{
+	LONG Active;
+	LONG UserAbortRequested;
+	LONG KeyDerivationAbort;
+	LONG UserSidValid;
+	LONG SequenceNumber;
+	int nDosDriveNo;
+	ULONG UserSidLength;
+	UCHAR UserSid[SECURITY_MAX_SID_SIZE];
+} MOUNT_CANCEL_CONTEXT, *PMOUNT_CANCEL_CONTEXT;
+
 
 /* This structure is allocated for non-root devices! WARNING: bRootDevice
    must be the first member of the structure! */
@@ -43,6 +59,7 @@ typedef struct EXTENSION
 	BOOL bThreadShouldQuit;		/* Instruct per device worker thread to quit */
 	PETHREAD peThread;			/* Thread handle */
 	KEVENT keCreateEvent;		/* Device creation event */
+	PMOUNT_CANCEL_CONTEXT MountCancelContext;
 	KSPIN_LOCK ListSpinLock;	/* IRP spinlock */
 	LIST_ENTRY ListEntry;		/* IRP listentry */
 	KSEMAPHORE RequestSemaphore;	/* IRP list request  Semaphore */