Block field names with dollar sign prefix at insert time

As reported in CXX-443, with MongoDB 2.6 and the right driver (in this case the 26compat C++ driver) it is possible to insert a document that contains a field prefixed with a dollar sign, whereas for a corresponding update the server returns an error:

"The dollar ($) prefixed field '$unwind' in 'pipeline.0.$unwind' is not valid for storage."

This seems inconsistent and unintuitive. The server should prevent dollar sign prefixed fields (and more generally, fields containing any reserved characters) from entering the system.