SRM: don't talk to SpaceManager when disabled

A recent patch (committed master@2ba5c9 and 2.2@d069ff) fixed an NPE.
The submitter mentioned concerns that this would "break something",
which turned out to be correct.  After the patch, SRM verbose ls
(and space-related operations) no longer function correctly if
space-manager is disabled.  This is because SRM verbose-ls always
sends a request to space-manager and, after the patch, space-manager
gives an error-response when running but configured to be disabled.

This patch alters the SRM so that if configuration shows that
space-manager is disabled then no communication with space-manager
is attempted.  SRM verbose ls will list all files are in no
space-reservations and the space-related operations will fail with
a friendly error message.

A side-effect of this patch is that the 'spacemanager' service is
no longer needed when space-manager is disabled.

Target: master
Request: 2.6
Request: 2.2
Patch: http://rb.dcache.org/r/5526/
Acked-by: Gerd Behrmann