shell: fix shell oracle for configuration keys with a space

The shell oracle is built to speed up POSIX shell querying of
configuration properties.  If the property key contains a
space then the resulting oracle is broken and prevents
any 'dcache' command from working.

Target: master
Patch: https://rb.dcache.org/r/7592/
Acked-by: Gerd Behrmann
Requires-book: no
Requires-notes: yes
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6