Introduce BOOST_FOREACH() helpers for LLSD in llsdutil.h.

You can't directly write:
BOOST_FOREACH(LLSD item, someLLSDarray) { ... }
because LLSD has two distinct iteration mechanisms, one for arrays and one for
maps, neither using the standard [const_]iterator typedefs or begin()/end()
methods. But with these helpers, you can write:
BOOST_FOREACH(LLSD item, llsd::inArray(someLLSDarray)) { ... }
or
BOOST_FOREACH(const llsd::MapEntry& pair, llsd::inMap(someLLSDmap)) { ... }
These are in namespace llsd instead of being (e.g.) llsd_inMap because with a
namespace at least your .cpp file can have a local 'using':
using namespace llsd;
BOOST_FOREACH(LLSD item, inArray(someLLSDarray)) { ... }
It's namespace llsd rather than LLSD because LLSD can't be both a namespace
and a class name.