daemon: Ignore '--keep-failed' for TCP/IP clients.

* nix/nix-daemon/nix-daemon.cc (performOp) <wopSetOptions>: When
'isRemoteConnection' is true, set 'settings.keepFailed' to zero.
* doc/guix.texi (Common Build Options): Document this behavior.

doc/guix.texi

@@ -6229,6 +6229,10 @@ the end of the build log.  This is useful when debugging build issues.
 @xref{Debugging Build Failures}, for tips and tricks on how to debug
 build issues.
 
+This option has no effect when connecting to a remote daemon with a
+@code{guix://} URI (@pxref{The Store, the @code{GUIX_DAEMON_SOCKET}
+variable}).
+
 @item --keep-going
 @itemx -k
 Keep going when some of the derivations fail to build; return only once

nix/nix-daemon/nix-daemon.cc

@@ -565,6 +565,12 @@ static void performOp(bool trusted, unsigned int clientVersion,
 
     case wopSetOptions: {
         settings.keepFailed = readInt(from) != 0;
+	if (isRemoteConnection)
+	    /* When the client is remote, don't keep the failed build tree as
+	       it is presumably inaccessible to the client and could fill up
+	       our disk.  */
+	    settings.keepFailed = 0;
+
         settings.keepGoing = readInt(from) != 0;
         settings.set("build-fallback", readInt(from) ? "true" : "false");
         verbosity = (Verbosity) readInt(from);