A few PAM cleanups

Print header found message only if libraries also found.
Change header guards to pragma once.
Check return value of store_client_password().

server/modules/authenticator/PAM/PAMAuth/pam_client_session.cc

@@ -358,8 +358,10 @@ bool PamClientSession::extract(DCB *dcb, GWBUF *buffer)
         break;
 
     case PAM_AUTH_DATA_SENT:
-        store_client_password(dcb, buffer);
-        rval = true;
+        if (store_client_password(dcb, buffer))
+        {
+            rval = true;
+        }
         break;
 
     default: