procstats.py: Collect interrupt statistics from /proc/interrupts

Collect per CPU interrupts statistics from /proc/interrupts.
Metric name: proc.interrupts
Interrupt types are classifed per "type" tag.
startstop: Use the current directory as default TCOLLECTOR_PATH

With the current directory as default TCOLLECTOR_PATH the startstop
script can be called in the tcollector root without setting
TCOLLETOR_PATH.
procstats.py: record per CPU values from /proc/stat

Updating procstats.py to use per-cpu breakdowns (cpu0, cpu1, etc)
instead of the aggregated "cpu" line. This works even in the case of
single-core boxes, which have identical cpu and cpu0 lines.

Also, I corrected the tag names for columns 8 and 9 to steal and guest as per proc(5).