support for appended and invariant SolrParams in solrconfig.xml

this patch would add support to StandardRequestHandler and DisMaxRequestHandler for specifying "appends" and "invariants" SolrParams in the solrconfig.xml, from the example config...

In addition to defaults, "appends" params can be specified
to identify values which whould be appended to the list of
multi-valu params from the query (or the existing "defaults").
...
"invariants" are a way of letting the Solr maintainer lock down
the options available to Solr clients. Any params values
specified here are used regardless of what values may be specified
in either the query, the "defaults", or the "appends" params.

...functionality is largely reusable by any handler, via SolrPluginUtils.setDefaults.