shell: rework code to use shell.spawn

spawn gives out streams and we have more control over the stdout/stderr
buffers. otherwise, we have to provide a max buffer capture size to exec

src/mailserver.js

@@ -53,8 +53,8 @@ async function generateDkimKey() {
     const privateKeyFilePath = path.join(os.tmpdir(), `dkim-${crypto.randomBytes(4).readUInt32LE(0)}.private`);
 
     // https://www.unlocktheinbox.com/dkim-key-length-statistics/ and https://docs.aws.amazon.com/ses/latest/DeveloperGuide/send-email-authentication-dkim-easy.html for key size
-    await shell.exec(`openssl genrsa -out ${privateKeyFilePath} 1024`, {});
-    await shell.exec(`openssl rsa -in ${privateKeyFilePath} -out ${publicKeyFilePath} -pubout -outform PEM`, {});
+    await shell.spawn('openssl', ['genrsa', '-out', privateKeyFilePath, '1024'], {});
+    await shell.spawn('openssl', ['rsa', '-in', privateKeyFilePath, '-out', publicKeyFilePath, '-pubout', '-outform', 'PEM'], {});
 
     const publicKey = safe.fs.readFileSync(publicKeyFilePath, 'utf8');
     if (!publicKey) throw new BoxError(BoxError.FS_ERROR, safe.error.message);