check-config: Produce error when using scoped properties

In the confusion of scoped properties, some sites have used the
scoping operator in their local configuration (probably not realizing
it was an operator and treating it as if it was part of the name).

When upgrading to 2.10, those sites will not receive any warnings or
errors about such definitions, as only the unscoped names are marked
obsolete/deprecated/forbidden. dCache does output an info level message
saying that dCache itself doesn't use the property, but sites tend
to ignore that message (based on a sample size of one).

This patch resolves this by flagging scoped properties as errors.
This prevents a slash from being used in a property name, but given
the history of scoped properties, it is probably wise to disallow
that symbol for some years.

Demo:

    $ packages/system-test/target/dcache/bin/dcache check-config
    [ERROR] dcache.conf:2: Property xrootd/xrootdPlugins is a scoped property.
                           Scoped properties are no longer supported.
    [INFO] system-test.conf:1: Property system-test.home is not a standard
                               property
    Found 1 error.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Require-notes: yes
Require-book: yes
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7401/
(cherry picked from commit f455fdc39c32619a996373cad3a00ffe1427e71a)
Merge pull request #983 from gbehrmann/fix/2.7/rb7401

check-config: Produce error when using scoped properties