Bring better error handling to inventory item and folder fetching.

First, introduced some LLSD-based interfaces to the llcorehttp code
using utils classes (in llcorehttputil).  I've kept LLSD out of
the llcorehttp library up to now and will continue to do that.
Functions provide a requestPost based on LLSD body and conversion
utils for HttpResponse-to-LLSD and HttpResponse-to-string
conversions.  Inventory fetch operations now do more thorough
error checking including 200-with-error status checking.  Still
do retry forever on folders though I don't like that.