Simplify EXEC catching + add signal handling

See merge request redox-os/syscall!51

I have had a think, and these flags are pointless. Since this is a
microkernel, there should always be exactly one way to do something, and
it should be the most flexible and simplest to implement. And you can
already just set breakpoints on the address provided by the signal
handler, and we'll give the entrypoint as an address to the fexec call.

Not entirely a useful thing, one *could* also put a breakpoint on the
handler address... I'll need to think about this some more.

change seek sig to match posix - signed pos and result

See merge request redox-os/syscall!45

ptrace: stop on fexec

See merge request !50

Add missing match item for `PartialAllocStrategy::Greedy`.

See merge request redox-os/syscall!48

Add a more complex physalloc syscall.

See merge request redox-os/syscall!46

ptrace: Block on read, not on write

See merge request redox-os/syscall!47

Allow Mmio to work with const fn

See merge request !43

Allow Dma<[T]>

See merge request !42

Final few ptrace changes

See merge request !41

Ptrace overhaul & bitflags

See merge request !40

Drive ptrace into a wall, prepare for overhaul

See merge request redox-os/syscall!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 :)