extmod/fsusermount: Change block protocol to support ioctl method.

The new block protocol is:
- readblocks(self, n, buf)
- writeblocks(self, n, buf)
- ioctl(self, cmd, arg)

The new ioctl method handles the old sync and count methods, as well as
a new "get sector size" method.

The old protocol is still supported, and used if the device doesn't have
the ioctl method.

py/qstrdefs.h

@@ -675,6 +675,7 @@ Q(readonly)
 Q(mkfs)
 Q(readblocks)
 Q(writeblocks)
+Q(ioctl)
 Q(sync)
 Q(count)
 #endif