cleanup: remove compiler warnings

Eliminate the cause of several diagnostics emitted by gcc 4.6.3 and
4.9.0:

- index, time, tmpfile, optind, and remove are all globally-visible
  symbols that were shadowed by local declarations
- time_t may be a signed type, resulting in a signed/unsigned comparison
  warning from an operation in rrd_xport
- bypass declarations of variables not referenced due to DISABLE_FLOCK
cleanup: fix link target
Merge pull request #472 from pabigot/cleanup-nowarnings

Cleanup nowarnings