Add -gp options to pstree(1).

I often use the -p option to get the pids of processes and missed this functionality in Sortix (I find pstree nicer to use than ps). I implemented the -g option to show the process group ids while here.

I'd also love the -a option to show the arguments of processes, but the kernel doesn't retain this information and there's no system call to get it, so that'll be another time.

I modernized the option parsing while here. The program remains undocumented per the time saving measure of not documenting programs that aren't new to Sortix that just behave like one would expect.