LP#1790923: disable XUL staff client by default

LP#1790923: disable XUL staff client by default

This patch discourages use of the legacy XUL staff client
by adjusting the installation process so that a versioned XUL
server directory is not created. Instead, by default XUL
server files (which is still needed by a few web staff client
interfaces) end up in /openils/var/web/xul/legacy. During installation
and upgrade, the /openils/var/web/xul/server symbolic link is
set to point to /openils/var/web/xul/legacy/server if possible.

If for some reason a given installation of Evergreen 3.2.x does not
wish to stop using the XUL staff client, STAFF_CLIENT_STAMP_ID
can still be provided during the 'make install' step, and the
make_release script can still create the XUL client installers
if given the -x switch.

Note, however, that use of the XUL staff client in 3.2.x is
NOT RECOMMENDED and no longer under any guarantee of community
support.

To test
-------
[1] Perform a fresh installation and verify that /openils/var/web/xul/legacy
    is created and that /openils/var/web/xul/server is a symlink
    pointing to /openils/var/web/xul/legacy/server.
[2] Verify that the web staff client works and that the
    user permissions editor in particular continues to work.
[3] Perform an upgrade; verify that /openils/var/web/xul/legacy exists
    and that if /openils/var/web/xul/server started out as a symlink,
    it has been repointed.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>