The standard _dup() function pattern takes a const-qualified pointer to an
object and returns a mutable copy. Add 'const' to a few that didn't follow
that pattern.

* subversion/include/svn_config.h,
  subversion/libsvn_subr/config.c
  (svn_config_dup): Add 'const' as above.

* subversion/include/svn_error.h,
  subversion/libsvn_subr/error.c
  (svn_error_dup): Add 'const' as above.

* subversion/libsvn_fs_base/dag.h,
  subversion/libsvn_fs_base/dag.c
  (svn_fs_base__dag_dup): Add 'const' as above.

* subversion/libsvn_fs_x/index.h,
  subversion/libsvn_fs_x/index.c
  (svn_fs_x__p2l_entry_dup): Add 'const' as above.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1585718 13f79535-47bb-0310-9956-ffa450edef68