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Additionally, the zircon syscall structs were out of date, causing us to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_fuchsia.rs

@@ -1,11 +1,13 @@
+use crate::convert::TryInto;
 use crate::io;
+use crate::fmt;
 use crate::mem;
 use crate::ptr;
 
 use crate::sys::process::zircon::{Handle, zx_handle_t};
 use crate::sys::process::process_common::*;
 
-use libc::size_t;
+use libc::{c_int, size_t};
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
@@ -160,7 +162,7 @@ impl Process {
             return Err(io::Error::new(io::ErrorKind::InvalidData,
                                       "Failed to get exit status of process"));
         }
-        Ok(ExitStatus::new(proc_info.rec.return_code))
+        Ok(ExitStatus(proc_info.return_code))
     }
 
     pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
@@ -190,6 +192,36 @@ impl Process {
             return Err(io::Error::new(io::ErrorKind::InvalidData,
                                       "Failed to get exit status of process"));
         }
-        Ok(Some(ExitStatus::new(proc_info.rec.return_code)))
+        Ok(Some(ExitStatus(proc_info.return_code)))
+    }
+}
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ExitStatus(i64);
+
+impl ExitStatus {
+    pub fn success(&self) -> bool {
+        self.code() == Some(0)
+    }
+
+    pub fn code(&self) -> Option<i32> {
+        // FIXME: support extracting return code as an i64
+        self.0.try_into().ok()
+    }
+
+    pub fn signal(&self) -> Option<i32> {
+        None
+    }
+}
+
+impl From<c_int> for ExitStatus {
+    fn from(a: c_int) -> ExitStatus {
+        ExitStatus(a as i64)
+    }
+}
+
+impl fmt::Display for ExitStatus {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "exit code: {}", self.0)
     }
 }