webdav: use '/' per user

Add Tigran's patch 532ddbf to 2.8 branch.

Conflicts:
	modules/dcache-webdav/src/main/java/org/dcache/webdav/DcacheResourceFactory.java
	modules/dcache-webdav/src/main/java/org/dcache/webdav/SecurityFilter.java
Not sure if the commented lines need to be in place of the other if clause below