Use exec on Unix to propagate signals

This is only possible when telemetry is disabled, but almost all of the time it
is! This should help fix lots of common rustup-related issues related to signals
and such.

Closes rust-lang#31
Closes rust-lang#806

src/rustup-cli/proxy_mode.rs

@@ -53,4 +53,3 @@ fn direct_proxy(cfg: &Cfg, arg0: &str, toolchain: Option<&str>, args: &[OsString
     };
     Ok(try!(run_command_for_dir(cmd, arg0, args, &cfg)))
 }
-

src/rustup/command.rs

@@ -21,7 +21,7 @@ pub fn run_command_for_dir<S: AsRef<OsStr>>(cmd: Command,
         return telemetry_rustc(cmd, arg0, args, cfg);
     }
 
-    run_command_for_dir_without_telemetry(cmd, arg0, args)
+    exec_command_for_dir_without_telemetry(cmd, arg0, args)
 }
 
 fn telemetry_rustc<S: AsRef<OsStr>>(mut cmd: Command,
@@ -132,7 +132,7 @@ fn telemetry_rustc<S: AsRef<OsStr>>(mut cmd: Command,
     }
 }
 
-fn run_command_for_dir_without_telemetry<S: AsRef<OsStr>>(
+fn exec_command_for_dir_without_telemetry<S: AsRef<OsStr>>(
     mut cmd: Command, arg0: &str, args: &[S]) -> Result<()>
 {
     cmd.args(args);
@@ -141,17 +141,20 @@ fn run_command_for_dir_without_telemetry<S: AsRef<OsStr>>(
     // when and why this is needed.
     cmd.stdin(process::Stdio::inherit());
 
-    match cmd.status() {
-        Ok(status) => {
-            // Ensure correct exit code is returned
-            let code = status.code().unwrap_or(1);
-            process::exit(code);
-        }
-        Err(e) => {
-            Err(e).chain_err(|| rustup_utils::ErrorKind::RunningCommand {
-                name: OsStr::new(arg0).to_owned(),
-            })
-        }
+    return exec(&mut cmd).chain_err(|| rustup_utils::ErrorKind::RunningCommand {
+        name: OsStr::new(arg0).to_owned(),
+    });
+
+    #[cfg(unix)]
+    fn exec(cmd: &mut Command) -> io::Result<()> {
+        use std::os::unix::prelude::*;
+        Err(cmd.exec())
+    }
+
+    #[cfg(windows)]
+    fn exec(cmd: &mut Command) -> io::Result<()> {
+        let status = cmd.status()?;
+        process::exit(status.code.unwrap());
     }
 }