ssh-store: add remote-store and remote-program query params
ssh-store: add remote-store and remote-program query params

Brings the functionality of ssh-ng:// in sync with the legacy ssh://
implementation.  Specifying the remote store uri enables various useful
things. eg.

    $ nix copy --to ssh-ng://cache?remote-store=file://mnt/cache --all

rycee/nix,ehmry/nix,NixOS/nix,ehmry/nix,NixOS/nix,ehmry/nix,rycee/nix,NixOS/nix,pikajude/nix,NixOS/nix,pikajude/nix,pikajude/nix,NixOS/nix,ehmry/nix,rycee/nix,rycee/nix,ehmry/nix,rycee/nix,ehmry/nix