Clean up Jvm.* subprocess/inprocess APIs

[lihaoyi]

They're a mess. #3770 makes them marginally more messy, but they've been messy for years and could use a cleanup.

Doing this properly would involve a lot of renaming and consolidation, which is binary incompatible and would need to wait until 0.13.0. If we have some ideas of how the APIs should look, we could put the "better" APIs into 0.12.0 and deprecate the others for removal later