Some IOCTLs are fixed, e.g. i2c-dev. There, we must not translate the
IOCTLs. Instead, the IOCTLs are generated arch-dependent in the fs
package.

- fs: add IO() IOR() IOW() IORW() functions for the equivalent linux macros.
- videocore: Replace ioctl magic with derived value.
  Beside being clearer, the comment in the code was incorrect. The message
  box magic is in fact 'd' = 100 = 0x64, not 0x100.
- videocore: Use dynamic pointer size for ioctl.