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
Merge pull request #1036 from PirataNervo/featurefix-936

Fixes #936 Wrong compatibility check for plugins
Fixes #369 Fixes #369 LIMIT clause in session updates

#369
Merge pull request #1038 from PirataNervo/featurefix-369

Fixes #369 LIMIT clause in session updates
Fixes #321 postsperpage negative value SQL error
Fixes #321 threadsperpage negative value SQL error
Merge pull request #1041 from PirataNervo/featurefix-321

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

https://github.com/mybb/mybb/issues/1025
Merge pull request #1043 from Destroy666x/fix1025

Fix #1025 feature
Merge pull request #884 from Nik101010/fix-836-feature

Fix: Length of guest usernames not checked [#836]
Merge branch 'feature' of https://github.com/mybb/mybb into feature