Make SSHMaster::startCommand work on an args list

This avoids split-on-whitespace errors:

- No more `bash -c` needed

- No more `shellEscape` needed

- `remote-program` ssh store setting also cleanly supports args (e.g.
  `nix daemon`)

- `ssh` uses `--` to separate args for SSH from args for the command to
  run.

and will help with Hydra dedup.

Some code taken from #6628.

Co-Authored-By: Alexander Bantyev <balsoft@balsoft.ru>

src/libstore/ssh.hh

@@ -41,7 +41,16 @@ public:
         AutoCloseFD out, in;
     };
 
-    std::unique_ptr<Connection> startCommand(const std::string & command);
+    /**
+     * @param command The command (arg vector) to execute.
+     *
+     * @param extraSShArgs Extra args to pass to SSH (not the command to
+     * execute). Will not be used when "fake SSHing" to the local
+     * machine.
+     */
+    std::unique_ptr<Connection> startCommand(
+        Strings && command,
+        Strings && extraSshArgs = {});
 
     Path startMaster();
 };