Change process spawning to inherit the parent's signal mask by default

Previously, the signal mask is always reset when a child process is
started. This breaks tools like `nohup` which expect `SIGHUP` to be
blocked.

With this change, the default behavior changes to inherit the signal mask.

This also changes the signal disposition for `SIGPIPE` to only be
changed if the `#[unix_sigpipe]` attribute isn't set.

library/std/src/rt.rs

@@ -89,7 +89,7 @@ macro_rules! rtunwrap {
 // `src/tools/tidy/src/pal.rs` for more info. On all other platforms, `sigpipe`
 // has a value, but its value is ignored.
 //
-// Even though it is an `u8`, it only ever has 3 values. These are documented in
+// Even though it is an `u8`, it only ever has 4 values. These are documented in
 // `compiler/rustc_session/src/config/sigpipe.rs`.
 #[cfg_attr(test, allow(dead_code))]
 unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {