The standard C lib strtoul signature is flawed in that it effectively
loses the const qualifier from the input buffer when returning the
updated "end" location.  No need to make the same mistake in our
variant of it.

* subversion/include/private/svn_string_private.h
  (svn__strtoul): make output constness match input constness

* subversion/libsvn_subr/string.c
  (svn__strtoul): update implementation

* subversion/libsvn_subr/types.c
  (svn_revnum_parse): update caller


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489824 13f79535-47bb-0310-9956-ffa450edef68