Temporary workaround for BSD WEXITSTATUS libc bug

The libc crate has a bug on BSD where WEXITSTATUS is not an 8-bit
value, causing assertion failures.

Any libc higher than our 0.2.155 would increase our MSRV, see libc
commit 5ddbdc29f (Bump MSRV to 1.71, 2024-01-07), so we want to
woraround this anyway.  It's probably not worth using a patched
version of libc since it's just one line.

While at it, tighten some types I guess.

Upstream fix: https://github.com/rust-lang/libc/pull/4213

Closes #10919

Cherry-picked from c1b460525c

CHANGELOG.rst

@@ -1,6 +1,7 @@
 Changes since 4.0b1
 -------------------
 - :kbd:`ctrl-c` cancels builtin ``read`` again, fixing a regression in the beta. :issue:`10928`
+- Fix a BSD-specific regression in the beta that caused crashes when a child process exited with a status like -1 (:issue:`10919`).
 - Fix a regression in the beta where ``__fish_cancel_commandline`` caused glitches on multi-line command lines (:issue:`10935`).
 - Self-installable builds can now also be installed to a specific location by giving a path to ``--install``, like::
     fish --install=$HOME/.local/

src/proc.rs

@@ -233,14 +233,14 @@ impl ProcStatus {
     }
 
     /// Return the exit code, given that we normal exited.
-    pub fn exit_code(&self) -> libc::c_int {
+    pub fn exit_code(&self) -> u8 {
         assert!(self.normal_exited(), "Process is not normal exited");
-        WEXITSTATUS(self.status())
+        u8::try_from(WEXITSTATUS(self.status()) & 0xff).unwrap() // Workaround for libc bug
     }
 
     /// Return if this status represents success.
     pub fn is_success(&self) -> bool {
-        self.normal_exited() && self.exit_code() == EXIT_SUCCESS
+        self.normal_exited() && self.exit_code() == u8::try_from(EXIT_SUCCESS).unwrap()
     }
 
     /// Return the value appropriate to populate $status.
@@ -248,7 +248,7 @@ impl ProcStatus {
         if self.signal_exited() {
             128 + self.signal_code()
         } else if self.normal_exited() {
-            self.exit_code()
+            i32::from(self.exit_code())
         } else {
             panic!("Process is not exited")
         }