Remove the requirement opaquelocktoken uri scheme

Remove the requirement opaquelocktoken uri scheme

The program flow of pushing over http is:

 - call lock_remote() to issue a DAV_LOCK request to the server to lock
   info/refs and branch refs being pushed into; handle_new_lock_ctx() is
   used to parse its response to populate "struct remote_lock" that is
   returned from lock_remote();

 - send objects;

 - call unlock_remote() to drop the lock.

The handle_new_lock_ctx() function assumed that the server will use a
lock token in opaquelocktoken URI scheme, which may have been an Ok
assumption under RFC 2518, but under RFC 4918 which obsoletes the older
standard it is not necessarily true.

This resulted in push failure (often resulted in "cannot lock existing
info/refs" error message) when talking to a server that does not use
opaquelocktoken URI scheme.

Signed-off-by: Kirill A. Korinskiy <catap@catap.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>