login: Fix empty input prompt

Issue #484.

PR392 inadvertantly replaced the `login` interactive process spawner
with
`child::run`, which is hard-coded to buffer stdout/stderr. This caused
`login` to become essentially unusable; the user could no longer see
interactive input prompts or error messages displayed by `npm adduser`.

The code was not directly reverted because the previous version:
1. Returned Error instead of failure::Error. (Updated to use
`bail!`, which is consistent with `publish`.)
2. Displayed all stderr only upon exit, rather than interactively
displaying it. This led to repeated interactive prompts without
informing the user why. (Updated to use `status()` which inherits
stdin/stdout/stderr by default.)
3. Did not provide logging. (Now duplicates the logging in
`child::run`.)

src/npm.rs

@@ -62,12 +62,14 @@ pub fn npm_login(
         args.push_str(&format!(" --auth_type={}", auth_type));
     }
 
+    // Interactively ask user for npm login info.
+    //  (child::run does not support interactive input)
     let mut cmd = Command::new("npm");
-    cmd.arg("login")
-        .arg(args)
-        .stdin(Stdio::inherit())
-        .stdout(Stdio::inherit());
-    child::run(log, cmd, "npm login")
-        .with_context(|_| format!("Login to registry {} failed", registry))?;
-    Ok(())
+    cmd.arg("login").arg(args);
+
+    info!(log, "Running {:?}", cmd);
+    match cmd.status()?.success() {
+        true => Ok(()),
+        false => bail!("Login to registry {} failed", registry),
+    }
 }