Improve input value checks in language editor

A PHP code injection vulnerability has been found in the ISPConfig language editor by Egidio Romano from Karma(In)Security. Thanks a lot for reporting this issue! The vulnerability requires that the attacker is correctly logged in as the 'admin' user (the account with superadmin privilege) in ISPConfig, so an attacker must know the administrator password. Not affected are logins from Clients, Resellers, or Email users and also not logins from additionally created admin users.

Also not affected are systems where the language editor is disabled. The language editor can be disabled by setting:

admin_allow_langedit=no

in the file /usr/local/ispconfig/security/security_settings.ini.