Rename legacy MS_* flags for ntfs_mount with NTFS_MNT_* flags.

The MS_* flags originated from system constants. However the flags
passed to ntfs_mount were really unrelated to the system constants and
many new MS_* flags had to be introduced as different features were
added to the library. Those flags had no counterparts in any system
APIs, so using the same naming scheme is inappropriate.

Instead, let's namespace these flags similarly to what has already been
done in ntfsprogs/libntfs earlier. This avoids any possible conflicts
with system constants.
The values of the flags themselves are kept the same as earlier, so
backward compatibility is retained.

ntfsprogs/ntfsck.c

@@ -850,7 +850,7 @@ int main(int argc, char **argv)
 	// at this point we know that the volume is valid enough for mounting.
 
 	/* Call ntfs_device_mount() to do the actual mount. */
-	vol = ntfs_device_mount(dev, MS_RDONLY);
+	vol = ntfs_device_mount(dev, NTFS_MNT_RDONLY);
 	if (!vol) {
 		ntfs_device_free(dev);
 		return 2;