Don't pass environment variables with "\n" to LightProcesses

A recent diff started (correctly) passing the environment
through to child processes. It turns out our LightProcess implementation
can't handle environment variables with new lines in them, so (for now)
strip out any such variables.

hphp/runtime/ext/ext_process.cpp

@@ -814,7 +814,10 @@ Variant f_proc_open(CStrRef cmd, CArrRef descriptorspec, VRefParam pipes,
       nvpair += iter.first().toString();
       nvpair += '=';
       nvpair += iter.second().toString();
-      envs.push_back(nvpair.detach().c_str());
+      string tmp = nvpair.detach().c_str();
+      if (tmp.find('\n') == string::npos) {
+        envs.push_back(tmp);
+      }
     }
 
     child = LightProcess::proc_open(cmd.c_str(), created, intended,