1) Refactor the manpage with num-options, optimize wording to be more
concise and to the point, pid also specifies process groups.
2) Make int sig const.
3) Remove prototypes.
4) /* not reached */ consistency.
5) Refactor usage() with eprintf.
6) Refactor arg-parser with a switch, use estrtonum
7) Use return instead of exit() in main()
8) argc-argv-correctness.
- add .Os, it is mandatory.
- don't redeclare .Nm when it's not needed.
- fix some warnings (checked with mandoc -Tlint).
- remove some leftover old stuff.
