Audit kill(1)

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.

README

@@ -37,7 +37,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =*  grep            yes                          none
 =*| head            yes                          none
 =*| hostname        non-posix                    none
-=*  kill            yes                          none
+=*| kill            yes                          none
 =*| link            yes                          none
 =*| ln              yes                          none
 =*| logger          yes                          none