Return quota information based on query path

[Vigilans]

This PR is the implementation of miquels/webdav-handler-rs#25.

It will append the queried path to self.basedir, so the quota query is based on the actual query path, instead of the root path.

Explanations from miquels/webdav-handler-rs#25 :

There are many cases when we use a folder like /media to export as webdav file root, but subfolders in /media are mounted with other filesystems:

/media -> rootfs
/media/dirA -> external ext4 system
/media/dirB -> external NTFS system

Currently, get_quota only collects quota information of root folder (the /media), even the client is querying quota information of path /media/dirA or /media/dirB.

When using clients like RaiDrive in Windows, it will report disk usage just like normal physical disks, and we expect it to show the correct disk usage of drive /media/dirA or /media/dirB (maybe some TBs large), but not the /media from rootfs `(maybe just tens of GBs).