Fixes to option parsing in kill, skill, pkill.

Hi,

These are some fixes to how kill, skill and pkill handle parameters in the -SIGNUM form.

The handling was incorrect in kill/skill, it was actually not properly truncating argc/argv before removing the -SIGNUM argument. There were some hacks around the code to work around the bug, but using kill -1 on its own would end up working as if kill -1 -1 (which means kill -HUP <all processes>) was executed. (Yes, it was painful when I accidentally typed it... more than once.)

I also made kill print the usage if it only gets a signal number, but no pids (as in the kill -1 example.)

And pkill had similar code but was using a buggy atoi() to try to parse the signal number, which meant that trailing garbage was accepted, so I corrected that by removing the buggy atoi() and letting the already existing code that already accepted numeric signals do its work as supposed to...

I also wanted to tackle the "pgrp" case where a negative number is passed to kill, causing it to kill a process group. The current code is buggy, for instance kill -TERM -2345 will kill process group 2 and not process group 2345 as supposed to. It should also be possible to pass it multiple pgrps or a mix of pgrps and pids. It's hard to fix that though, considering how getopt_long() works, so I'll defer that for a second pull request.

I tested this fairly well, both manually and made sure there were no regressions in the test suite, also didn't break make distcheck.

Let me know if you have any questions or other remarks...

Cheers! Filipe