Fixed #4173 No SSL Tab for client if SSL available but also Let\'s Encrypt available template limit is active.

- Changed default value for expire of ftp user, should have fixed: #3862

- added limit for letsencrypt and directive snippets selector to client limits
- hide ssl tab if letsencrypt is selected

- backported patches

- http://en.wikipedia.org/wiki/SPDY

- Directive snippets can now be made available to clients; clients can select an available directive snippet for a website - the appropriate snippet will then be written to the vhost configuration file. This is useful for example on nginx where clients have no access to the Options tab of a website.

FS#3632 - Adding websites with remote API is broken in 3.0.5.4p3 release.
Missed to commit one file for: FS#3634 - Add option to disable statistics for a website

- Added warning in the interface when a path for a shelluser is set that is outside of the website docroot.
- Added security settings feature to allow the root user of a server to control most aspects of whet the admin user of the controlpanel is allowed to do in system settings. This is especially useful for managed severs where the ispconfig admin user and the root user of the server are different persons.