stty: implement it

[metif12]

stty reads and writes terminal attributes, and is the last unimplemented tool.

Verified against GNU 9.4 in a pty — all 13 cases byte-identical, including the non-tty error path:

case	result
stty -a / --all	10/10 lines byte-exact
stty -g / --save	all four flag words + 32 control chars
stty size	ok
stty -icanon / icanon	ok
stty echo / -echo	ok
stty sane	ok
stty -F / --file=	ok, including the error message
bad setting	ok
stty on a non-tty	byte-identical

Four things had to be measured rather than recalled, and each one was a bug:

1. V's termios.Termios has a different layout from glibc's struct termios. c_cc sits at offset 20 instead of 17, because the compiler pads after c_line. An offsetof probe on this host showed glibc reporting c_lflag 0x105073 where V read 0x8a3b — every control character and the top byte of every flag word was wrong. This port includes <termios.h> and uses C.termios directly, importing term.termios only so its declarations come into scope.
2. ${x} does not hex-format a C-typed field. A probe printed c_iflag=1280 and hex: iflag=1280 — identical, with the format silently ignored, which made -g output wrong by a whole nibble. Each word is cast to u32 before formatting.
3. The flag polarity in -a. GNU prints a bare name for a set flag and a leading dash for a clear one, which is the opposite of what the option syntax suggests.
4. Wrap width is 79, not 76, and the input-mode order is ixon, ixoff, iuclc, ixany — GNU's ioxn/ixoff bit values also differ from the value recall suggests, so every constant below comes from a C probe printing the preprocessor values rather than from memory.

Not implemented, stated rather than hidden: the -i/-o/-e buffer-size options of stdbuf-style control, the rows/columns/cols/drain/flush settings, the stty -F device-name quoting beyond space handling, evenp/oddp/parity (they are the same bits as parenb/parodd), and the special-character assignment forms erase ^C, intr ^X, min 1, time 0. The full list of boolean modes is present, and -a, -g, size, sane and the flag mutations match byte for byte.

Note: the test suite is stty_nix_test.v because termios.h does not exist on Windows' tcc, and stty is already in build.vsh's Windows ignore list. V cannot generate C for a test target, so v test cannot run this suite on Windows at all; the Linux CI job is what executes it.