Drive ptrace into a wall, prepare for overhaul

See merge request !39

Signals now cause an event, and there's a way to continue until the
next signal. I can see this being used for detection of `int3`
although I'm not entirely sure as it may prove being too late to stop
abortion of process.

Rust does not allow a `fn`-pointer to be null. This fixes that, while
luckily doing it in a way that leaves system calls
backwards-compatible :)

Fix floating-point struct

See merge request !37

- deprecate chmod
- add tests
- CI using redoxer

Bare-bones ptrace functionality

See merge request !36