Fix: Length of guest usernames not checked, refs #836

* inc/datahandlers/post.php
  * added verification of username for guests input using user datahandler
* newreply.php / newthread.php
  * added verification of username for preview
* inc/languages/english/datahandler_post.php
  * added language strings for possible error messages in post author validation
Fixes #936 Wrong compatibility check for plugins
Fixes #369 Fixes #369 LIMIT clause in session updates

#369
Fixes #321 postsperpage negative value SQL error
Fixes #321 threadsperpage negative value SQL error
Fix #1025 feature

https://github.com/mybb/mybb/issues/1025