Remove a const cast by duplicating a string in the ra_serf sspi code.
Remove a const cast by duplicating a string in the ra_serf sspi code.

* subversion/libsvn_ra_serf/win32_auth_sspi.c
  (sspi_get_credentials): Add scratch_pool argument.
  (sspi_maxtokensize): Add scratch_pool argument and apr_pstrdup
     the passed argument.
  (svn_ra_serf__init_sspi_connection): Update caller.
  (do_auth): Update caller.
  (sspi_get_credentials): Add scratch_pool argument and pass it to sspi_maxtokensize.
  (svn_ra_serf__init_proxy_sspi_connection): Update caller.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion