without the need to destroy the session, if the sharee is a different user
This is accomplished by keeping the sharee in the stream context / attribute of the vfs class(es) instead of static Vfs::$user.
Later is still used for the current user - identical to egw_info[user][account_id].
This commit / merge of the vfs-context feature branch also added the abilty to mount WebDAV sources, eg. a sharing link from an other EGroupware instance
or any WebDAV server
