Move some functionality to a utils module (#162)
* Move some functionality to a utils module
This code will be shared with the authorized keys command, which will be built as a separate binary.
* Time parsing and lint fixes.
* gofmt updates
* Rename google_guest_utils to utils
* Return errors from utils functions
* Update a couple of missed calls.
* Fix some formatting errors.
* Fixes from comments
* Update logging to log bad keys once to the user
This restores "badExpire" for windows_accounts and diagnostics, and adds logging of all bad keys for SSH keys.
* gofmt fixes
