[PATCH] SMP Cache manager: add support for parameterized Cache Manager queries

The attached patch add support for parameterized Cache Manager queries.

Currently, one sends mgr queries to the "whole" Squid. Kids responses
may get aggregated by Coordinator, and we, in general, want to aggregate
all responses that can be aggregated.

This patch allow us to aggregate stats for a subset of kids. For
example, the following query aggregates stats for just the first and the
third workers:

     mgr:info?workers=1,3

When query response information cannot be aggregated (or at least is not
aggregated right now), then a parameterized query will result in several
matching "byKid { ..." blocks.

This patch support the following scope variants:

     * raw interface with access to any kid process or groups of kids;
similar to ${process_number} macro we already support in squid.conf:
           mgr:foo?processes=id,id,id...
     * higher-level interface to isolate workers by their numbers,
starting with 1 for the first worker:
          mgr:foo?workers=num,num,num...

Currently, all kids except Coordinator are workers, but that will change
in the future as we get more kinds of kids.Currently, one sends mgr
queries to the "whole" Squid. Kids responses may get aggregated by
Coordinator, and we, in general want to aggregate all responses that can
be aggregated.