setterm: use getopt_long_only() for option parsing

The getopt_long_only() has advantage of allowing one and two hyphen
versions of options, for example -help and --help.  Secondly the getopt
function family from libc can ensure some options requiring arguments,
while other are optional, and some should not accept argument at all.
That makes option parsing more robust.

Unfortunately retiring the old option parsing makes this change greater
than preferred.  Assuming not mistakes happen the new code works
functionally exactly as the old did.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
setterm: use string utils to numeric parsing

Check the input numbers are numbers, which makes also the code shorter,
and user experience better as half invalid imputs will error.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
setterm: move show_tabs() and screendump() functions

Earlier the function was in the middle of option parsing code segment,
and screendump() required function prototype.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
setterm: remove usage comment segment

It is enough trouble to keep usage output aligned with getopt() parsing,
manual page, and bash-completion, so get rid of the unnecessary effort
duplication.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
setterm: add option control structure

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