setup filesystem by username, not login name, fixes #11637

User names and login names may differ with other user backends. If they differ, login via webdav was broken, because the login name was passed which caused the user not to be found and setup fail.

This can be tested properly using integration tests, only.

This must get into 7.0.3!

Please review @PVince81 @icewind1991 @DeepDiver1975 @davivel

cc @craigpg @karlitschek