webdav: implement memLS.Confirm.

LockSystem.Confirm now takes two names (a src and a dst), and returns a
func() instead of a 1-method Releaser.

We now pass 25 of 30 "locks" litmus tests:
- 4 failures are due to PROPFIND / PROPPATCH not being implemented.
- 1 failure is due to shared locks being unsupported.
- 3 warnings are also due to PROPFIND / PROPPATCH.
- 1 failure (cond_put_corrupt_token) is due to returning 412
  (Precondition Failed) instead of 423 (Locked), but IIUC
  the spec says to return 412.
- 11 tests were skipped, presumably due to earlier failures.

Change-Id: I3f4c178cdc4b99c6acb7f59783b4fd9b94f606ec
Reviewed-on: https://go-review.googlesource.com/3860
Reviewed-by: Dave Cheney <dave@cheney.net>
3 files changed