I renamed `tty?` to `terminal?`.  As of now, there are no instances of "tty" in the SRFI except in the command `stty` and the names of Posix functions.  In DEC OSes, device names couldn't be more than three letters long, but that's long dead.

I have concluded that control of symlinks does meet the 80-20 limitation, especially as implementation is trivial.  So I have added back `create-symlink`, `read-symlink`, and `file-info-symlink?`.

I've also merged hga up to date.  Unless there are major objections to the substantive changes, I'll do a PR for Arthur (it'll take him a while to get to it; he's swamped).