See [strace-redox#ea42589d](redox-os/strace-redox@ea42589d)

This is a curious problem and it's really hard to solve it in a way
that doesn't feel hacky. On one hand, of course you want to be able to
modify and intercept what happens when you use a signal, right? On the
other hand, changes made to the context (especially singlestepping)
while a signal is handled (such as `SIGSTOP`) are not preserved since
the stack is restored after the signal handler was invoked.

I think what we have in this change makes sense anyway, as we don't
really want users modifying registers and other data in the default
signal behavior that occurs **in kernel mode**. Also trying to use
`PTRACE_SINGLESTEP` will set the singlestep flag only if in a
user-mode signal handler, else it will set it on the instruction after
the signal handling, which I guess makes sense since it can't affect
the kernel-mode code that runs the default handler.

I don't know. Help. Pls.

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 :)

Ptrace memory reading and floating point registers support

See merge request !104

Since even a very basic ptrace can be nice to have, I thought I would split
the, perhaps rather big, ptrace project up in multiple PRs to make as few
changes as necessary in each. This PR contains the initial registry modifying
bits and only a very basic security measure. Letting this out to the community
should be good for spotting bugs and maybe getting some hype ;)

This reverts merge request !103

Bare-bones ptracing functionality

See merge request redox-os/kernel!103

I'm guessing it's some issue after a rebase or something...

Switch to 2018 edition

See merge request !102

Most of this was generated by the absolutely extraordinary `cargo fix`
subcommand. There were still 2 errors and a few warnings to patch up,
but compared to the normal 600+ errors, I'd say the fixer did a damn
good job! I'm also amazed that I could still start the VM after this,
I half expected some kinds of runtime failure...

Fix dropping grants without unmapping if they have no file descriptor (such as those from anonymous maps)