Begin extending spillbufs so that we can use them for compressed pristines.

* subversion/libsvn_subr/spillbuf.c (svn_spillbuf_t):
   Add new fields delete_on_close, spill_all_contents, dirpath and temp_path.
  (init_spillbuf_extended): New. Initializes a spillbuf.
  (init_spillbuf): New. Initializes a spillbuf, defaulting the new fields.
  (svn_spillbuf__create): Use init_spillbuf.
  (svn_spillbuf__reader_create): Likewise; do away with the cut-and-paste
   initialization silliness.
  (svn_spillbuf__write): Use the new svn_spillbuf_t fields when creating
   the temporary spill file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509080 13f79535-47bb-0310-9956-ffa450edef68