tty_write is relatively short and the only function left in tty-write.c so
move it into tty.c.

More tty code tidying: move the saved cursor/region position (from before the
screen was updated) out of struct screen and into struct tty_ctx.

enum tty_cmd is only used as an index into the array of command function
pointers, so remove it and use the function pointers directly to represent
themselves.

There are relatively few arguments to tty_cmd_* functions now, so tidy them up
by using a struct rather than hiding everything with varargs.

decision for whether or not a pane should be drawn out of the layout code and
into the redraw code.

This is needed for the new layout design, getting it in now to make that easier
to work on.

duplicate. Found by lint.