l4::utcb::Registers: implement modes

[humenda]

The UTCB should be writable or readable, but not both at the same time.
This invariant ensures that data that being read is still valid, except a call
happened in between.
An example: reading an Array is safe, i.e. it's a pointer to the UTCB with a
length. Writing an array is safe as long as the Array is not part of the UTCB.
Differenciating the UTCB in read and write attaches a lifetime to each reference
that exists to it and this one is dropped once the mode is changed.