[loader] Parse in more netcore hosting properties

[CoffeeFlux]

This includes some other minor cleanup to the netcore hosting functions.

Setting assemblies_path with APP_PATHS despite it also being set by MONO_PATH doesn't scare me too much because:

1. Anyone using MONO_PATH is probably not going through the hosting API and is using mono-sgen directly
2. The default host doesn't set APP_PATHS unless Microsoft.NETCore.DotNetHostPolicy.SetAppPaths is set to 1 in the .runtimeconfig.json (which is rare, and we've worked fine without APP_PATHS being set at all up until now)
3. This is a temporary measure; we will have to refactor our preload hook when we want to properly support the other loader properties anyway

[vitek-karas]

Nit about APP_PATHS: the host will set it if the .runtimeconfig.json contains property Microsoft.NETCore.DotNetHostPolicy.SetAppPaths set to 1. Almost nobody does that, so I don't think it matters - but just to make thing complete 😇

[CoffeeFlux]

Ah good to know, thanks! I agree it's still not a concern, but worth keeping in mind - I'll update the description to be more accurate.