Fix ExitStatus on Fuchsia

Fuchsia exit codes don't follow the convention of libc::WEXITSTATUS et
al, and they are 64 bits instead of 32 bits. This gives Fuchsia its own
representation of ExitStatus.

Additionally, the zircon syscall structs were out of date, causing us to
see bogus return codes.

src/libstd/sys/unix/process/process_unix.rs

@@ -1,3 +1,4 @@
+use crate::fmt;
 use crate::io::{self, Error, ErrorKind};
 use crate::ptr;
 use crate::sys::cvt;
@@ -441,3 +442,54 @@ impl Process {
         }
     }
 }
+
+/// Unix exit statuses
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitStatus(c_int);
+
+impl ExitStatus {
+    pub fn new(status: c_int) -> ExitStatus {
+        ExitStatus(status)
+    }
+
+    fn exited(&self) -> bool {
+        unsafe { libc::WIFEXITED(self.0) }
+    }
+
+    pub fn success(&self) -> bool {
+        self.code() == Some(0)
+    }
+
+    pub fn code(&self) -> Option<i32> {
+        if self.exited() {
+            Some(unsafe { libc::WEXITSTATUS(self.0) })
+        } else {
+            None
+        }
+    }
+
+    pub fn signal(&self) -> Option<i32> {
+        if !self.exited() {
+            Some(unsafe { libc::WTERMSIG(self.0) })
+        } else {
+            None
+        }
+    }
+}
+
+impl From<c_int> for ExitStatus {
+    fn from(a: c_int) -> ExitStatus {
+        ExitStatus(a)
+    }
+}
+
+impl fmt::Display for ExitStatus {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(code) = self.code() {
+            write!(f, "exit code: {}", code)
+        } else {
+            let signal = self.signal().unwrap();
+            write!(f, "signal: {}", signal)
+        }
+    }
+}