Fix for https://github.com/owncloud/core/issues/7516

Link to issue:
https://github.com/owncloud/core/issues/7516
only allow alphanumeric characters and "_" in name for installation subdirectory
The change (obviously) also fixes the problems with "/" and "?" characters described here:
https://forum.owncloud.org/viewtopic.php?f=23&t=9824#p24418

It's quite locked down now but these chars should definitely be safe and it also enforces readable directory names.