Cleanup old RustPOSIX concurrency mechanisms in RawPOSIX

[rennergade]

We now use futex() to handle most concurrency primitives. Because of this we can get rid of the outdated implementations for mutex() and cvs that RawPOSIX inherited from RustPOSIX.

For now we want to leave semaphores as they are because of their interaction with shared memory, but we'll have to tackle that in the near future.

[rennergade]

This is a most likely a two step issue.

Step 1:

• Remove all the mutex and cv data structures and functions from RawPOSIX that are left over from RustPOSIX/nacl
• Add a futex table similar to the ones that were removed for mutex and cv
• Add functionality in fork so that futexes are cloned on fork
• Add tests for this

Step 2:

• After shared memory is fixed we'll need to fix the semaphore functions