Refactor fs/host.TTYFileOperations so that the relevant functionality can be
shared with VFS2 (fsimpl/host.ttyFD).
Incorporate host.defaultFileFD into the default host.fileDescription. This way,
there is no need for a separate default_file.go. As in vfs1, the TTY file
implementation can be built on top of this default and override operations as
necessary (PRead/Read/PWrite/Write, Release, Ioctl).
Note that these changes still need to be plumbed into runsc, which refers to
imported TTYs in control/proc.go:ExecAsync.
Updates #1672.
PiperOrigin-RevId: 301718157
Dean Deng