Following up on r1552324, copy the settings hash in the ra session
implementations that now store a reference to the whole config. Tweak
documentation and move public function.

* subversion/include/svn_ra.h
  (svn_ra_dup_session): Move function from between the svn_ra_openX()
    functions and extend documentation a bit.

* subversion/libsvn_ra_serf/serf.c
  (svn_ra_serf__open,
   ra_serf_dup_session): Copy config hash instead of just referencing it.

* subversion/libsvn_ra_svn/client.c
  (open_session): Copy config hash instead of just referencing it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552894 13f79535-47bb-0310-9956-ffa450edef68