kill: add '--' option to separate options from arguments

kill: add '--' option to separate options from arguments

Using a negative pid to send TERM to a process group results in an
obscure error:

   $ ./busybox kill -12345
   kill: bad signal name '12345'

This is intended. Manpage says:
ARGUMENTS
       pid    Each pid can be one of four things:
...
              -n     where n is larger than 1.  All processes in process group
                     n are signaled.  When an argument of  the  form  '-n'  is
                     given,  and it is meant to denote a process group, either
                     a signal must be specified first, or the argument must be
                     preceded  by a '--' option, otherwise it will be taken as
                     the signal to send.

However, we did not support "--". Add this capability to BusyBox.

function                                             old     new   delta
kill_main                                            993     999      +6

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>