Support for per-task I/O statistics - $top_io
Basically, I just added three new process properties (io_read, io_write, io_perc - representing
the amount of I/O done by the process during the update interval) and $top_io, that sorts
processes based on io_perc.
Atm, it's completely #ifdef'd, since it requires kernel support. But that creates some wierd
looking syntax at some places, so it may be better to remove some ifdefs. It even may be
possible to completely remove the ifdefs (ie. convert them to #ifdef linux) since the code will
compile just fine even if the kernel doesn't support I/O accounting. I'll leave that for someone
else to decide.