irq plugin: Fixed some issues, renamed RRD-files.

irq plugin: Fixed some issues, renamed RRD-files.

I've looked into your plugin in more detail just now and fixed the
following issues:
- Moved the RRD-files into a subdirectory called `irq'. That's how it's
  going to be done in the next major version.
- Renamed the DS to `value' and set the maximum value to 65535. I'm not
  sure if this maximum value is big enough..?
- Don't use `LOG_EMERG' when config-problems are found. Rather, print to
  `STDERR' since it's still available during configuration.
- Use `strsplit' when parsing `/proc/interrupts'. This makes much of the
  pointer-arithmetic obsolete and the code much more readable.
- The device names are not used in the filename, though I'm not certain
  if that's the way to go here.. On my workstation there is a line like
  this:
    193:      28492   IO-APIC-level  uhci_hcd, uhci_hcd, uhci_hcd, ehci_hcd
  Apparently the interrupt is shared among several USB-controllers. My
  problem here is, that as soon as one device is deactivated or the
  order changes, so does the filename. Besides, a filename along the
  lines of `irq-193-uhci_hcd-ehci_hcd.rrd' would be nice.
  On another machine I have this line:
    217:   50173955          0   IO-APIC-level  3ware Storage Controller
  Where one device somehow manages to write a _description_ in there.
  This makes detection of such shared interrupts as above tricky.
  If anyone has a good idea how to use the last part of the lines for a
  filename, please let me know.