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/provision.js

@@ -60,7 +60,7 @@ function setProgress(task, message) {
 async function ensureDhparams() {
     if (fs.existsSync(paths.DHPARAMS_FILE)) return;
     debug('ensureDhparams: generating dhparams');
-    const dhparams = await shell.exec('openssl dhparam -dsaparam 2048', {});
+    const dhparams = await shell.spawn('openssl', ['dhparam', '-dsaparam', '2048'], { encoding: 'utf8' });
     if (!safe.fs.writeFileSync(paths.DHPARAMS_FILE, dhparams)) throw new BoxError(BoxError.FS_ERROR, `Could not save dhparams.pem: ${safe.error.message}`);
 }