Simplification/refactoring of key/file processing

Simplification/refactoring of key/file processing

This is a fairly major overhaul to greatly reduce the number of
redundant code paths.  We here created a new process_keys_for_file
function that processes key from a userid for a given key file.  All
the main top elevel functions now call this one function.

The main top level monkeysphere functions for updating the user's
authorized_keys and known_hosts files are now moved to their own
sourced files, which greatly reduces the amount of code sourced with
common.

monkeysphere now updates authorized_keys and known_hosts in temporary
files that are then atomically moved into place upon completion.

Finally, removed the confusing return codes in the key/file processing
functions that were based on number of valid/invalid keys processed.
It was confusing in the presence of actual errors that stopped
processing.