Support all-or-nothing mode when locking multiple files

The following question came from Eric Miller <Eric.Miller@amd.com>:

   Is there a way to get svn_client_lock to do an all-or-nothing lock?
   Right now when it encounters an error during lock the function gives
   a warning but keeps locking.

   For instance:

     user1% svn lock a
     user2% svn lock a b
     svn: warning: Path '/trunk/a' is already locked by user 'user1 in
filesystem '/svn/test/db'
     'b' locked by user 'user2.

   Is this a bug or intended behavior?   I can see this being useful for
   collections of binary data.

Answers included this from me:

   Nope.  Locks aren't protected by atomicity guarantees outside what is
   required to maintain the integrity of the backend storage.  The
   repository APIs only permit locking one path at a time, and no other
   layer of interface between client and repository currently attempts to
   corral multiple path-lock attempts under the umbrella of all-or-nothingness.

And this from Ben Collins-Sussman:

   Yeah, this use-case just didn't cross our minds when we designed the
   feature.  It's certainly a feature that could be added in the future,
   if there were enough demand.

http://svn.collab.net/viewvc/svn/branches/issue-2699-dev