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/atexit/wsgi.py

@@ -2,7 +2,8 @@ import atexit
 
 def application(environ, start_response):
     def at_exit():
-        environ['wsgi.errors'].write('At exit called.')
+        environ['wsgi.errors'].write('At exit called.\n')
+        environ['wsgi.errors'].flush()
 
     atexit.register(at_exit)