[WIP] Update to nix 0.17.

[ericonr]

Use killpg(pid, sig) instead of kill(-pid, sig).

---

Hi! The current nix version (0.8) in this library is broken for ppc targets. I would like to update to the latest version (0.17), but from release 0.9 onwards they removed the From<nix::Error> for std::io::Error trait impl: nix-rust/nix#614

Therefore, when compiling, I get these errors:

    Checking unsegen v0.2.4 (/home/ericonr/mypro/ugdb/unsegen)
error[E0277]: `?` couldn't convert the error to `std::io::Error`
  --> src/base/terminal.rs:87:77
   |
87 |         pthread_sigmask(SigmaskHow::SIG_UNBLOCK, Some(&stop_and_cont), None)?;
   |                                                                             ^ the trait `std::convert::From<nix::Error>` is not implemented for `std::io::Error`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following implementations were found:
             <std::io::Error as std::convert::From<nix::errno::Errno>>
             <std::io::Error as std::convert::From<std::ffi::NulError>>
             <std::io::Error as std::convert::From<std::io::ErrorKind>>
             <std::io::Error as std::convert::From<std::io::IntoInnerError<W>>>
   = note: required by `std::convert::From::from`

error[E0277]: `?` couldn't convert the error to `std::io::Error`
  --> src/base/terminal.rs:90:35
   |
90 |         killpg(getpgrp(), SIGTSTP)?;
   |                                   ^ the trait `std::convert::From<nix::Error>` is not implemented for `std::io::Error`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following implementations were found:
             <std::io::Error as std::convert::From<nix::errno::Errno>>
             <std::io::Error as std::convert::From<std::ffi::NulError>>
             <std::io::Error as std::convert::From<std::io::ErrorKind>>
             <std::io::Error as std::convert::From<std::io::IntoInnerError<W>>>
   = note: required by `std::convert::From::from`

error[E0277]: `?` couldn't convert the error to `std::io::Error`
  --> src/base/terminal.rs:95:75
   |
95 |         pthread_sigmask(SigmaskHow::SIG_BLOCK, Some(&stop_and_cont), None)?;
   |                                                                           ^ the trait `std::convert::From<nix::Error>` is not implemented for `std::io::Error`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following implementations were found:
             <std::io::Error as std::convert::From<nix::errno::Errno>>
             <std::io::Error as std::convert::From<std::ffi::NulError>>
             <std::io::Error as std::convert::From<std::io::ErrorKind>>
             <std::io::Error as std::convert::From<std::io::IntoInnerError<W>>>
   = note: required by `std::convert::From::from`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.
error: could not compile `unsegen`.

To learn more, run the command again with --verbose.

How would you suggest we go about restructuring the code to allow it to work with newer nix versions?

[ftilde]

I've just pushed a fix to your PR branch (I hope you don't mind!). Fortunately, I don't think we ever encounter errors other than the "standard" errno in the calls that we make here, so we are/should be fine just assuming they don't happen (and gracefully crash if they do). The alternative would be to return nix::Errors instead, but that would be a breaking change. Anyways, I don't see other issues with this patch, so thank you for that!

[ericonr]

No worries, thanks for the fix!