stty: Finish '--save' support

[dezgeg]

Argument parsing for this exists, but the option doesn't change output in any way. Finish the support.

Closes #3862.

I haven't found any documentation on the format of this or looked at any code, but it would appear to be simple hex printing of some (not all) struct termios fields:

$ cat termiostest.c
#include <stdio.h>
#include <termios.h>

int main() {
    struct termios t;
    tcgetattr(0, &t);
    while (1)
        ;
}
$ gcc -g termiostest.c
$ gdb ./a.out
(gdb) run
(gdb) p/x t
$1 = {c_iflag = 0x100, c_oflag = 0x5, c_cflag = 0xbf, c_lflag = 0x8a3b, c_line = 0x0, c_cc = {0x3, 0x1c, 0x7f, 0x15, 0x4, 0x0, 0x1, 0x0, 0x11, 0x13, 0x1a, 0x0, 0x12, 0xf, 0x17, 0x16, 0x0 <repeats 16 times>},
  c_ispeed = 0xf, c_ospeed = 0xf}
$ stty --save
100:5:bf:8a3b:3:1c:7f:15:4:0:1:0:11:13:1a:0:12:f:17:16:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail-2/inotify-dir-recreate

[tertsdiepraam]

Nice! Yeah I couldn't find any docs either, that's why I left it out of my original implementation. This looks good though :)

[tertsdiepraam]

We should set up some way to test stty too, but that's out of scope for this PR. On my local machine, the output seems to match up with GNU. Great work!