value-pairs: Allow comma-separated scopes in the command-line parser

Allow comma-separated scopes in the command-line parser, in
$(format-json) and anything else that may be using the same framework.
We do this by splitting the value by commas first, and iterating through
the list.

Fixes: BugZilla#248
Reported-by: Fabien Wernli <bugzilla.balabit@faxm0dem.org>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>