ssh-ng: Set log-fd for ssh to 4 by default

That's expected by `build-remote` and makes sure that errors are
correctly forwarded to the user.

For instance, let's say that the host-key of `example.org` is unknown
and

    nix-build ../nixpkgs -A hello -j0 --builders 'ssh-ng://example.org'

is issued, then you get the following, somewhat cryptic error:

    error: cannot open connection to remote store 'ssh-ng://example.org': error: unexpected end-of-file

The relevant information (`Host key verification failed`) ends up in the
daemon's log, but that's not very obvious considering that the daemon
isn't very chatty normally.

This can be fixed - the same way as its done for legacy-ssh - by passing
fd 4 to the SSH wrapper. Now you'd get the following error:

    error: cannot open connection to remote store 'ssh-ng://example.org': error: unexpected end-of-file: Host key verification failed.

...and now it's clear what's wrong.

src/libstore/machines.cc

@@ -61,10 +61,10 @@ ref<Store> Machine::openStore() const
     Store::Params storeParams;
     if (hasPrefix(storeUri, "ssh://")) {
         storeParams["max-connections"] = "1";
-        storeParams["log-fd"] = "4";
     }
 
     if (hasPrefix(storeUri, "ssh://") || hasPrefix(storeUri, "ssh-ng://")) {
+        storeParams["log-fd"] = "4";
         if (sshKey != "")
             storeParams["ssh-key"] = sshKey;
         if (sshPublicHostKey != "")

src/libstore/ssh-store.cc

@@ -25,6 +25,8 @@ class SSHStore : public virtual SSHStoreConfig, public virtual RemoteStore
 {
 public:
 
+    const Setting<int> logFD{(StoreConfig*) this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
+
     SSHStore(const std::string & scheme, const std::string & host, const Params & params)
         : StoreConfig(params)
         , RemoteStoreConfig(params)
@@ -38,7 +40,8 @@ class SSHStore : public virtual SSHStoreConfig, public virtual RemoteStore
             sshPublicHostKey,
             // Use SSH master only if using more than 1 connection.
             connections->capacity() > 1,
-            compress)
+            compress,
+            logFD)
     {
     }