improve "User is disallowed to alter" messages

some messages are very clumsy & should be improved :

./api/classes/class.cuser.php: self::exception(ZBX_API_ERROR_PARAMETERS, _s('User is disallowed to alter GUI access to him self by linking to user group "%s".', $group['name']));
./api/classes/class.cuser.php: self::exception(ZBX_API_ERROR_PARAMETERS, _s('User is disallowed to alter system status to him self by linking to user group "%s".', $group['name']));

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suggestions :
"User may not modify <GUI access|system status> for himself by becoming a member of user group "%s".', $group['name']."

general style improvements, avoid "linking" when talking about group membership (that's reserved for templates).