Big big refactor. Probably should have been multiple changes. - Makes FFI safer by explicitly specifying c_int in calls. - Uses ToPrimitive/FromPrimitive to make this easier. - Pulls PamFlag variables into a bitflags! struct. - Pulls PamMessageStyle variables into an enum. - Renames ResultCode to ErrorCode. - Switches from PAM_SUCCESS to using a Result<(), ErrorCode>. - Uses thiserror to make ErrorCode into an Error. - Gets rid of pam_try! because now we have Results. - Expands some names (e.g. Conv to Conversation). - Adds more doc comments. - Returns passwords as a SecureString, to avoid unnecessarily keeping it around in memory.