Some varbuf enhancements:

- Introduce new ap_varbuf_pdup() and ap_varbuf_regsub() functions.
- Fix some bugs in ap_varbuf_strmemcat().
- Make ap_varbuf.buf point to an empty string if no buffer has been allocated,
  yet.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1176018 13f79535-47bb-0310-9956-ffa450edef68