SERVER-12369 SERVER-11461 Update mongodump and mongorestore to properly handle users and roles in 2.6

There are some known caveats:
(1) It's not safe to run mongorestore while user-management commands are
executing on a live system (SERVER-12539).

(2) To restore 2.4 users to a 2.6 system that has never had any users added, you
must manually insert the version document indicating v1 users into
admin.system.version, which must be done by a highly privileged user, before
beginning the restore (SERVER-12541).