Minor tweaks for API consistency.

* subversion/include/svn_auth.h
  (svn_auth_cleanup_func_t): Was svn_auth_cleanup_callback.
  (svn_auth_cleanup_walk): Track type change, rename 'cleanup' to
    'cleanup_func', and update docstring for a bit more clarity.

* subversion/libsvn_subr/auth.h
  Track type and parameter name changes.  Also, employ the same
  internal preprocessor stuffs and formatting thereof that other
  header files use (__cplusplus, SVN_<libname>_<filename>).

* subversion/libsvn_subr/auth.c,
* subversion/libsvn_subr/simple_providers.c
  Track type and parameter name changes.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1465053 13f79535-47bb-0310-9956-ffa450edef68