Support arbitrary store URIs in nix.machines

For backwards compatibility, if the URI is just a hostname, ssh://
(i.e. LegacySSHStore) is prepended automatically.

Also, all fields except the URI are now optional. For example, this is
a valid nix.machines file:

  local?root=/tmp/nix

This is useful for testing the remote build machinery since you don't
have to mess around with ssh.

src/libstore/store-api.hh

@@ -668,20 +668,31 @@ void removeTempRoots();
 /* Return a Store object to access the Nix store denoted by
    ‘uri’ (slight misnomer...). Supported values are:
 
-   * ‘direct’: The Nix store in /nix/store and database in
+   * ‘local’: The Nix store in /nix/store and database in
      /nix/var/nix/db, accessed directly.
 
    * ‘daemon’: The Nix store accessed via a Unix domain socket
      connection to nix-daemon.
 
+   * ‘auto’ or ‘’: Equivalent to ‘local’ or ‘daemon’ depending on
+     whether the user has write access to the local Nix
+     store/database.
+
    * ‘file://<path>’: A binary cache stored in <path>.
 
-   If ‘uri’ is empty, it defaults to ‘direct’ or ‘daemon’ depending on
-   whether the user has write access to the local Nix store/database.
-   set to true *unless* you're going to collect garbage. */
-ref<Store> openStore(const std::string & uri = getEnv("NIX_REMOTE"));
+   * ‘https://<path>’: A binary cache accessed via HTTP.
 
-ref<Store> openStore(const std::string & uri, const Store::Params & params);
+   * ‘s3://<path>’: A writable binary cache stored on Amazon's Simple
+     Storage Service.
+
+   * ‘ssh://[user@]<host>’: A remote Nix store accessed by running
+     ‘nix-store --serve’ via SSH.
+
+   You can pass parameters to the store implementation by appending
+   ‘?key=value&key=value&...’ to the URI.
+*/
+ref<Store> openStore(const std::string & uri = getEnv("NIX_REMOTE"),
+    const Store::Params & extraParams = Store::Params());
 
 
 void copyPaths(ref<Store> from, ref<Store> to, const PathSet & storePaths, bool substitute = false);