add an autoreject_file parameter to the master config file

This acts like autosign_file, allowing one to set up a blacklist for new/pending keys which overrides both auto_accept and the autosign_file.

This is my first time modifying any of the client auth code, so I am not certain that I am returning the correct values and building the load for the auth events that are fired. This may need to be corrected before merging.