Support screen size change notifications

Telnet, RLogin, and SSH all have a mechanism for the client to notify the server when the terminal size changes. These should be supported by Synchronet. Would help avoid the need to periodically re-check as discussed in #529.

assigned to @Deuce

It appears this is already support for telnet (via the NAWS negotiation).

Ah, but it only updates telnet_cols and telnet_rows, not cols and rows.

A JS callback should be created for event-driven scripts as well... for the rest, possibly a "changed" flag that can be cleared by the script would work?

When this happens, the current position, line counter, etc. all become unknown... we'll likely need to query at least the cursor position.

Some terminals send many resize events when dragging as well, so processing may need to be deferred in some way.