refactor: add ioctl/mmap callbacks to fs_node_t, decouple ioctl dispatch

refactor: add ioctl/mmap callbacks to fs_node_t, decouple ioctl dispatch

Added ioctl and mmap function pointers to fs_node_t for generic
device dispatch. Refactored syscall_ioctl_impl to call node->ioctl
instead of hardcoding TTY/PTY dispatch by inode number.

- tty.c: added tty_devfs_ioctl wrapper, set on console/tty nodes
- pty.c: added pty_slave_ioctl_fn wrapper, set on all slave nodes
- syscall.c: ioctl now dispatches generically through node callback

This prepares the VFS for /dev/fb0 and other devices that need
ioctl and mmap support.