Disable the code that loads the system certificate store.

This was far from working cross-platform, so instead of trying to use the system's store, the user now has to supply the set of trusted certificates. At a later time, vibe.d might possibly distribute its own trusted store (e.g. based on Mozilla's one).
Improve HTTPClient's certificate validation handling.

By default, no validation is done to keep backwards compatibility until a proper trusted certificate store is usable by default. At the same time, a user callback can be set to change the validation mode and to add trusted certificates manually.
Add a note about the location of the system's CA store on Linux.