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>

Update collectd, base of Luci statistics, to 5.5.0.

Patches have been refreshed.
write-graphite plugin is now enabled. (see #1351)

I have compile-tested all plugins with ar71xx,
and real-life tested the following plugins:
Conntrack, Processor (CPU), DNS, Interfaces, Wireless, System Load,
Memory, Ping, Uptime

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

-update collectd to the current release 5.4.1
-fix sigrok glib check (upstream patch after 5.4.1)

Note:
Changes are required also to luci-statistics in Luci source.
Patch in http://patchwork.openwrt.org/patch/5303/

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>