kill: make usage() not to return

And change the function argument to be an output stream.  Earlier the
--help option made kill exit with none-zero value, that is now corrected.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
kill: deprecat invocation as 'pid' command name

Enabling options by renaming command is both unexpected and undocumented.
This magic is now deprecated and with remark of removal of this
functionality in future.

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>