pnfsmanager,doors: Add user relative upload directories

This patch adds user relative upload directories. If dcache.user-directory
is defined to a relative path, it is now interpreted relative to the
account's root directory. This works around the problem that there may
not be a common directory accessible through all accounts. Rather than
adding extra FTP doors, this patch allows extra upload directories to
be created.

The downside is that the upload directories may clash with other items
in the users directory. Also, the user could bypass the access limitations
(if he got write permission to the parent of the upload directory).
Finally, the sysadmin has several directories to check for orphaned
temporary upload directories.

All doors and pnfsmanager needs to be updated for the change to be effective.
If the feature is not used, then there is no limitation on upgrades.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7328/
(cherry picked from commit 2976ef12952b86800f91bc8740577eff385997ee)