Add atomic initializatoin function which doesn't return error and doesn't use
pools svn_atomic__init_once_void().

This is never yet used but is a groundwork to add xdigest checksum backend
which does require initialization. This initilization will be performed in
errorless context (inside of svn_checksum__sha1_ctx_create for example).

* include/private/svn_atomic.h
  (svn_atomic__void_init_func_t): New callback declaration.
  (svn_atomic__init_once_void): New function.

* libsvn_subr/atomic.c
  (init_baton_t): Add void_init_func field.
  (void_init_func_wrapper): Add function to wrap initilization in case of
   init_once_void.
  (svn_atomic__init_once_void): Implement function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1932028 13f79535-47bb-0310-9956-ffa450edef68