Reduce config overhead for use of socache interface by allowing

Reduce config overhead for use of socache interface by allowing
default paths to be used if none are configured:

* include/ap_socache.h (ap_socache_provider_t::create):
  Allow arg to be NULL to force use of defaults.
  (ap_socache_provider_t::init): Rename 'namespace' parameter to
  'cname' and restrict to allow use in filesystem paths.

* modules/cache/mod_socache_dbm.c (socache_dbm_create,
  socache_dbm_init),
  modules/cache/mod_socache_shmcb.c (socache_shmcb_create,
  socache_shmcb_init):
  Default to use of runtimedir-relative paths if no
  explicit path is configured.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@724682 13f79535-47bb-0310-9956-ffa450edef68