std: implement sendfile on linux

[terinjokes]

This changset adds a sendfile(2) syscall bindings to the linux bits
component. Where available, the sendfile64(2) syscall will be
transparently called.

A wrapping function has also been added to the std.os to transform
errno returns to Zig errors.

[daurnimator]

Should also add a prototype in std/c.zig so that libc's function is used when linking against libc.

[daurnimator]

This should just be an unconditional system.sendfile. If someone attempts to call this function on a system without it then they will get a compile error indicating as much.

[terinjokes]

@daurnimator I had added it to c.zig locally, but building anything linking against C fails with an unrelated error, so I can't test if my changes are good.

[terinjokes]

/usr/lib/zig/std/os.zig:3301:60: error: cast discards const qualifier
    switch (errno(system.setsockopt(fd, level, optname, opt.ptr, @intCast(socklen_t, opt.len)))) {

[terinjokes]

Per discussion on IRC, this PR can likely be extended with macOS and FreeBSD support:

• Extend the os.zig wrapper to accept hdr, sbytes, and flags parameters.
• Handle writing headers and trailers on systems where this isn't the default. Document what systems they're written atomically.
• The BSD and Linux handle updates to the offset pointer different, so I may need to also document the behavior and what would be portable.

Windows has TransmitFile as part of mswsock, which looks similar. Will have to investigate farther.

[andrewrk]

I'm surprised (and disappointed) this error is even possible; the kernel should be able to make this "just work" even if it is forced to use a small stack-based buffer due to heap memory pressure.

By convention, ENOMEM in zig turns into one of these two things:

• error.OutOfMemory if it is userspace heap memory that is exhausted
• error.SystemResources if it is kernel space memory or other resources that are unavailable.

In this case I believe it would be error.SystemResources.

[shawnl]

Reading through the kernel sources, this actually comes from files that have a mandatory lock, and the underlying splice() cannot return ENOMEM.

[terinjokes]

@shawnl Thanks, I can update the comment. I'll likely take @andrewrk's recommendation of using error.SystemResources in this case.

[terinjokes]

I've pushed updates to the Linux support to accept the wider *BSD parameters, and to send the header and trailer iovecs. I doubt I have implemented the behavior to match correctly.

Next up to to add support sendfile(2) from FreeBSD libc. Then I'll be in a better place to verify the behavior is the same.

[terinjokes]

I need to double check that the offset parameters here should be u64 instead of off_t.

[shawnl]

I don't think zig should support 32-bit off_t, in which case they are the same, but yeah probably still use off_t even if it must be u64.

[andrewrk]

Thanks! This is merged into #4611.

[terinjokes]

Sorry for not getting this as polished as I liked. Thank you for taking the time to carry it through.