Unit application library.

Library now used in all language modules.
Old 'nxt_app_*' code removed.

See src/test/nxt_unit_app_test.c for usage sample.

test/python/close/wsgi.py

@@ -8,4 +8,5 @@ class application:
         yield b''
 
     def close(self):
-        self.environ['wsgi.errors'].write('Close called.')
+        self.environ['wsgi.errors'].write('Close called.\n')
+        self.environ['wsgi.errors'].flush()