Code duplication shelluser_jailkit_plugin

I think shelluser_jailkit_plugin() should actually extend shelluser_base_plugin() to reduce code duplication. Which the name 'base' already seems to suggest.

Comparing them with vim -d server/plugins-available/shelluser_base_plugin.inc.php server/plugins-available/shelluser_jailkit_plugin.inc.php I noticed a number of tiny differences that have accumulated already.

I've started with syncing some of the trivial changes.