Add TIOCGWINSZ support.

Needed to print a histogram. Might also be handy for other output
methods later.
Finish implementing histograms.

I'm not sure this is going to stick around since rendering
is fairly inconsistent and totally dependent on the behavior
of the client terminal and font configuration.

That said, it's kinda cool.
Use syscall.Syscall.

Rawsyscall goes straight to the kernel, while Syscall() will
coordinate with the goroutine scheduler. It's not a big deal
for pcstat but looks cleaner so here we are.