wcfs: Setup basic logging for warnings/errors to go to stderr when invoked as e.g. `wcfs serve`

If we do not setup logging explicitly, it will print only

    No handlers could be found for logger "wcfs"

instead of emitting useful details in before e.g.

    RuntimeError: fuse_unmount /dev/shm/wcfs/<X>: failed: fusermount: failed to unmount /dev/shm/wcfs/<X>: Device or resource busy
    (more details logged)

-> Fix it.

wcfs/__init__.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2021  Nexedi SA and Contributors.
+# Copyright (C) 2018-2022  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -737,6 +737,10 @@ def main():
     zurl = argv[-1]     # -a -b zurl    -> zurl
     optv = argv[:-1]    # -a -b zurl    -> -a -b
 
+    # setup log.warn/error to go to stderr, so that details could be seen on
+    # e.g. "fuse_unmount: ... failed (more details logged)"
+    logging.basicConfig(stream=sys.stderr, level=logging.WARNING)
+
     if cmd == "serve":
         if argv[0] == '-h':
             os.execv(_wcfs_exe(), [_wcfs_exe(), '-h'])