[feat] Implement missing TTY termios ioctls (TCSETSF, TCSETA*)

[fslongjin]

Summary

Complete TTY termios-related ioctl handling so that tcsetattr() and legacy termio paths behave like Linux 6.6.

Observed behavior

During apt install, user space reports:

E: Setting in Start via TCSAFLUSH for stdin failed! - tcsetattr (25: Inappropriate ioctl for device)

This corresponds to tcsetattr(fd, TCSAFLUSH, ...) → ioctl(TCSETSF).

Root cause

In kernel/src/driver/tty/tty_core.rs, tty_mode_ioctl() currently handles only:

• TCGETS
• TCSETS (TCSANOW)
• TCSETSW (TCSADRAIN)

The following ioctls are defined but not wired; requests hit the default branch (ENOIOCTLCMD), which surfaces to user space as ENOTTY:

ioctl	value	typical user API
TCSETSF	0x5404	tcsetattr(..., TCSAFLUSH)
TCSETA	0x5406	tcsetattr (struct termio)
TCSETAW	0x5407	same + drain
TCSETAF	0x5408	same + flush

Expected behavior

Align with Linux 6.6 TTY termios ioctl semantics, including flush-before-set for TCSETSF / TCSETAF and correct handling of struct termio for the TCSETA* family.

Acceptance criteria

• tcsetattr(0, TCSAFLUSH, &t) succeeds on a serial console (nographic) shell
• The debconf error above no longer appears during apt install (except when stdin is legitimately not a TTY)
• TCSETA / TCSETAW / TCSETAF do not return ENOTTY on a valid TTY fd
• Covered by c_unitest (e.g. extend existing TTY tests such as test_tty_tcflush.c)