Sigset conveniance

[JonathanWoollett-Light]

Adds convenience functions for dealing with SigSet.

let mut sigset = SigSet::empty();
sigset.add(SIGINT);
pthread_sigmask(SigmaskHow::SIG_SETMASK, Some(&sigset), None)

becomes

let sigset = SigSet::from(SIGINT);
pthread_sigmask(SigmaskHow::SIG_SETMASK, Some(&sigset), None)

---

let mut sigset = SigSet::empty();
sigset.add(SIGINT);
sigset.add(SIGUSR1);
sigset.wait();

becomes

(SIGINT | SIGUSR1).wait();

[rtzoeller]

Seems worthwhile to add a changelog note about this. Otherwise looks good, thanks!

[JonathanWoollett-Light]

@rtzoeller Added change logs notes.

[JonathanWoollett-Light]

@rtzoeller Could you please re-review.

[asomers]

I don't know what happened here, but this sure is a riotous merge error. Could you fix it please?

[JonathanWoollett-Light]

Fixed.

[SteveLauC]

### Addded has already been added:

### Added
- Added `impl From<Signal> for SigSet`.
  ([#1959](https://github.com/nix-rust/nix/pull/1959))
- ...

### Changed
- ...

### Added
- Added `Icmp` and `IcmpV6` to `SockProtocol`.
  (#[2103](https://github.com/nix-rust/nix/pull/2103))

There are now two ### Added sections in this file, we need to merge them

[SteveLauC]

### Addded has already been added:

### Added
- Added `impl From<Signal> for SigSet`.
  ([#1959](https://github.com/nix-rust/nix/pull/1959))
- ...

### Changed
- ...

### Added
- Added `Icmp` and `IcmpV6` to `SockProtocol`.
  (#[2103](https://github.com/nix-rust/nix/pull/2103))

There are now two ### Added sections in this file, we need to merge them

[SteveLauC]

Except for that duplicate CHANGELOG section, this PR LGTM

BTW, nice improvement!

[JonathanWoollett-Light]

@SteveLauC

### Addded has already been added:

Yeah but I didn't add the 2nd one, there where 2 ### Added sections before this PR.

[SteveLauC]

Yeah but I didn't add the 2nd one, there where 2 ### Added sections before this PR.

Get it, would you like to fix them in this PR? Merging them as-is still LGTM, then I will fix it in another PR

---

The current CHANGELOG.md mode is indeed bad, making contributing experience suck, I hope we can switch to towncrier as soon as possible...

[JonathanWoollett-Light]

Yeah but I didn't add the 2nd one, there where 2 ### Added sections before this PR.

Get it, would you like to fix them in this PR? Merging them as-is still LGTM, then I will fix it in another PR

The current CHANGELOG.md mode is indeed bad, making contributing experience suck, I hope we can switch to towncrier as soon as possible...

I think it would make sense to fix this in a specific PR.

[SteveLauC]

Thanks!