internal package Foswiki::Configure::Checkers::Certificate::KeyChecker 
Checker for Certificate Key items.
This checker validates files that contain private key files
such as for the S/MIME signatures and for the Tasks framework.
It must be subclassed for the various certificate types, as the requirements
are slightly different.
  ObjectMethod check( $keys, $password, $reporter ) 
Validates a Key item for the configure GUI 
-  $keys- configure value object
-  $password- Optional password key name for this file
A lot of checking is done here to prevent mystery errors at runtime.