Add linux support for sched_attr and related syscalls

[matthri]

Hi,
I this PR should add support for the sched_attr config struct and the related sched_getattr and sched_setattr syscalls.
Since this is my first PR to this repo and I'm not that experienced with the different standard libraries I encountered some problems and wanted to ask for some assistance/ hints on how this best implemented in the interests of this project.

1. Header file conflict: The sched_attr struct is defined in linux/sched/types.h but when I add this file to the includes in the build.rs file I get some build errors since the file additionally defines the sched_param struct which is also defined by the bits/types/struct_sched_param.h header which is included by pthread.h.
   Can you point me in the right direction how this is best resolved?
2. Multiple struct versions: There exist multiple versions of the sched_attr struct which are distinguished by the size field of the struct.
   Whats in your opinion the best way to represent this in this crate?

Many thanks in advance and sorry for the wall of text above :D

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[JohnTitor]

The libc crate is a thin wrapper, and if glibc doesn't expose it, it shouldn't be added.

[matthri]

Thanks for your response.
Before opening this PR I had seen that there are already some methods in the android target which are not exposed and use the syscall directly, so I hoped this might also be applicable here.

E.g.

libc/src/unix/linux_like/android/b32/x86/mod.rs

Lines 591 to 615 in e077dbf

	f! {
	// Sadly, Android before 5.0 (API level 21), the accept4 syscall is not
	// exposed by the libc. As work-around, we implement it as raw syscall.
	// Note that for x86, the `accept4` syscall is not available either,
	// and we must use the `socketcall` syscall instead.
	// This workaround can be removed if the minimum Android version is bumped.
	// When the workaround is removed, `accept4` can be moved back
	// to `linux_like/mod.rs`
	pub fn accept4(
	fd: ::c_int,
	addr: *mut ::sockaddr,
	len: *mut ::socklen_t,
	flg: ::c_int
	) -> ::c_int {
	// Arguments are passed as array of `long int`
	// (which is big enough on x86 for a pointer).
	let mut args = [
	fd as ::c_long,
	addr as ::c_long,
	len as ::c_long,
	flg as ::c_long,
	];
	::syscall(SYS_socketcall, SYS_ACCEPT4, args[..].as_mut_ptr())
	}
	}

[JohnTitor]

Note that it's just a temporal workaround, I mean it should also be removed but it isn't now.

[matthri]

I understand, thanks again for your time.