Follow-up r1618641.  Give our strtol replacement a more appropriate
name, check for another overflow case and improve the commentary.

* subversion/libsvn_fs_fs/id.c
  (fast__strtol):  Rename to ...
  (locale_independent_strtol): ... this and improve (see above).
  (part_parse,
   txn_id_parse, 
   svn_fs_fs__id_parse): Update callers.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618653 13f79535-47bb-0310-9956-ffa450edef68