Follow-up to r1483292:  Document that the core function does not include
any overflow checking.  Add overflow checks to svn_revnum_parse.

Found by: Mattias Engdegård <mattiase@bredband.net>

* subversion/include/private/svn_string_private.h
  (svn__strtoul): note missing overflow checks in docstring plus give
                  advice how to handle that situation

* subversion/libsvn_subr/types.c
  (svn_revnum_parse): limit revnums to signed 32 bits (the only portable
                      limit we have on them) and 10 digits


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483535 13f79535-47bb-0310-9956-ffa450edef68