Add engine.stop() to stop all apps.

src/README.md

@@ -258,6 +258,10 @@ following keys are recognized:
    printed.
 
 
+— Function **engine.stop**
+
+Stop all apps in the engine by loading an empty configuration.
+
 — Function **engine.now**
 
 Returns monotonic time in seconds as a floating point number. Suitable

src/core/app.lua

@@ -137,6 +137,12 @@ function configure (new_config)
    counter.add(configs)
 end
 
+
+-- Stop all apps by loading an empty configuration.
+function stop ()
+   configure(config.new())
+end
+
 -- Removes the claim on a name, freeing it for other programs.
 --
 -- This relinquish a claim on a name if one exists. if the name does not
@@ -721,6 +727,11 @@ function selftest ()
    main({duration = 4, report = {showapps = true}})
    assert(app_table.app3 ~= orig_app3) -- should be restarted
 
+   -- Check engine stop
+   assert(not lib.equal(app_table, {}))
+   engine.stop()
+   assert(lib.equal(app_table, {}))
+
    -- Check one can't unclaim a name if no name is claimed.
    assert(not pcall(unclaim_name))