The updater is required to survive beyond termination of the viewer that
launched it so it can launch the next installer, or a replacement viewer.
Having the old viewer forcibly terminate it on shutdown would be counter-
productive.

Introduce a third LLLeap::create() overload taking LLProcess::Params, which
gives access to autokill, cwd and other options previously unsupported by
LLLeap. Reimplement the existing create() overloads in terms of this new one,
since LLLeapImpl::LLLeapImpl() is already based on LLProcess::Params anyway.

Use LLProcess::Params in LLAppViewer::init() to specify the updater process,
setting autokill=false.

Refactoring LLLeapImpl() apparently involved engaging an LLInitParam::Block
feature never before used: had to drag operator() into Multiple from its base
class TypedParam (as has been done in other TypedParam subclasses).