Auto merge of #97802 - Enselic:add-no_ignore_sigkill-feature, r=joshtriplett

Support `#[unix_sigpipe = "inherit|sig_dfl"]` on `fn main()` to prevent ignoring `SIGPIPE`

When enabled, programs don't have to explicitly handle `ErrorKind::BrokenPipe` any longer. Currently, the program

```rust
fn main() { loop { println!("hello world"); } }
```

will print an error if used with a short-lived pipe, e.g.

    % ./main | head -n 1
    hello world
    thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', library/std/src/io/stdio.rs:1016:9
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

by enabling `#[unix_sigpipe = "sig_dfl"]` like this

```rust
#![feature(unix_sigpipe)]
#[unix_sigpipe = "sig_dfl"]
fn main() { loop { println!("hello world"); } }
```

there is no error, because `SIGPIPE` will not be ignored and thus the program will be killed appropriately:

    % ./main | head -n 1
    hello world

The current libstd behaviour of ignoring `SIGPIPE` before `fn main()` can be explicitly requested by using `#[unix_sigpipe = "sig_ign"]`.

With `#[unix_sigpipe = "inherit"]`, no change at all is made to `SIGPIPE`, which typically means the behaviour will be the same as `#[unix_sigpipe = "sig_dfl"]`.

See https://github.com/rust-lang/rust/issues/62569 and referenced issues for discussions regarding the `SIGPIPE` problem itself

See the [this](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Proposal.3A.20First.20step.20towards.20solving.20the.20SIGPIPE.20problem) Zulip topic for more discussions, including about this PR.

Tracking issue: https://github.com/rust-lang/rust/issues/97889

compiler/rustc_session/src/config.rs

@@ -36,6 +36,8 @@ use std::iter::{self, FromIterator};
 use std::path::{Path, PathBuf};
 use std::str::{self, FromStr};
 
+pub mod sigpipe;
+
 /// The different settings that the `-C strip` flag can have.
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum Strip {
@@ -798,7 +800,15 @@ impl UnstableOptions {
 // The type of entry function, so users can have their own entry functions
 #[derive(Copy, Clone, PartialEq, Hash, Debug, HashStable_Generic)]
 pub enum EntryFnType {
-    Main,
+    Main {
+        /// Specifies what to do with `SIGPIPE` before calling `fn main()`.
+        ///
+        /// What values that are valid and what they mean must be in sync
+        /// across rustc and libstd, but we don't want it public in libstd,
+        /// so we take a bit of an unusual approach with simple constants
+        /// and an `include!()`.
+        sigpipe: u8,
+    },
     Start,
 }