libpurple: re-add user dir function, fix input requests

- Input requests were a horrible hack: a bunch of prpl-specific logic was
  in there for no discernible reason.
- This has been ripped out, and now it just forwards the primary string to
  the user, and feeds whatever they reply back to the prpl (with feedback!)
- An admin user (for the purposes of input requests) is now always added with
  the user 'spectrum2-libpurple-admin', instead of some weird prpl-specific
  name.
- Also, for stateful prpls like the tdlib one, you can now set the user dir.
  You really shouldn't set the user dir if you can help it, especially not
  in a multi-user setup, but now you can if you really need to.