kill: align with howto-usage-function.txt

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: use libc error printing facilities and exit values

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill, procs: fix coding style

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill, procs: move function declarations to kill.h

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill, procs: use pid_t for pids

Reference: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/threads.h#n30
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add long options and tidy up kill.1

Signed-off-by: Sami Kerola <kerolasa@iki.fi>