On the authzperf branch: Add a stringbuf helper function.

* subversion/include/svn_string.h
  (svn_stringbuf_leftchop): New prototype.
* subversion/libsvn_subr/string.c
  (svn_stringbuf_leftchop): Implement.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/authzperf@1618869 13f79535-47bb-0310-9956-ffa450edef68