irq plugin: Implement collection of non-numeric interrupts.

Hi all!

I've created a patch that enables collectd to collect all irqs present in
/proc/interrupts, with numeric and string names (NMI, LOC, RES, etc).

Two notices:
a) Unfortunately I was unable to build collectd from git repo (build.sh
   failures), therefore I created this patch for latest stable release: 4.10.1
b) I am fairly unexperienced in C programming, so please inform me gently of my
   mistakes:) But I am already using a patched version and it works fine.

Any feedback on code quality or recommendations are welcome.

Change-Id: Ia17bd953921051dfe53e838b76672adbef9e73c3
Signed-off-by: Florian Forster <octo@collectd.org>