PSR-2 for usermanager plugin

replace some var keywords

This should fix some tests on PHP 7.1. The work is far from done, we
still have a lot of variables without proper accessibility defined.

removed implementation of _getcsv() in usermanager

Our minimum PHP version requirement is now over 5.3, so this workaround is
not needed anymore. See also c9454ee3a82b29418a7ee59f0bb35006ea6a7fb1

Per FS#2884, implement a local version of str_getcsv() to maintain
compatibility with php 5.2.x (str_getcsv() is only available in
php 5.3+ and is used by user manager import feature.

test import against auth plugin which doesn't allow adding users

Add unit tests for usermanager import & export functions