HPCC-2566 Allow attribute values to be specified on the command line

Also allow ^ as a synonym for the root scope.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
If I define a symbol A.B via -D, does that "hide" any other module A that might be around? If so, is that desirable? If not, should it?