kill: leeway deprecation of --pid invocation as command name

Who knows if anyone is linking or copying kill to a file name 'pid' and
using --pid option functionality that way, so lets inform these users and
get rid of command name hack after few releases.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: use control structure to pass user input to functions

This should make it easier to understand how the program works.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: tell what is wrong rather than output usage()

Getting usage as error message is not specific enough.  As a user I want
to know what is wrong, and if it is unclear after error message how to
recover then I run command with --help.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: add parse_arguments() function

Long main() is difficult to read, so moving argument parsing to separate
function should make sense.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: move sigqueue inputs to control struct

Use of global variables is messy.  The earlier implementation also
assumed queue argument never to be textual, such as 'HUP', which now
works as one might expect.

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