Update collectd, the backbone of Luci statistics, to 5.5.1
Refresh patches. Main changes:
- Remove 500-upstream-parallel-build-fix.patch (implemented upstream)
- Add 600-fix-libmodbus-detection.patch to revert an upstream change
that broke libmodbus detection in collectd's configure script
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
