Fix (and centralize) temporary directory creation in the http server.

Fix (and centralize) temporary directory creation in the http server.

* httpd/api.cxx (build_collection_rh::POST): Always ensure there is a
  server temporary directory (otherwise files are attempted to be written
  to the root directory). Use make_temp_directory() to create the client
  temporary directory.
* httpd/utils.cxx (make_temp_dir): New function.
* httpd/utils.h: Added make_temp_dir() prototype.
* httpd/server.cxx (connection_info::postdataiterator): Use
  make_temp_directory().