default path, admin rights and user install

[ibilon]

This issue https://github.com/HaxeFoundation/haxe/issues/5328 reminded me of a couple of issues I have with the default path.

First, the one mentioned in the linked issue, it shouldn't be a sub-directory of haxe. It's already hard enough to change haxe version.

Second, we shouldn't expect people to run haxelib/haxe with admin/sudo rights, or go around it by making the haxelib folder 755 or things like that. We should differentiate between global and user install, just like pypy/npm/gem/...

On a single user computer it's no big deal (with the exception of the ridiculous access rights errors you can get from using a system dir), but if it were to be installed on a university computer network for instance it wouldn't work.