Add support for terminal resizing for exec, attach, and run. Note that for Docker, exec sessions
inherit the environment from the primary process, so if the container was created with tty=false,
that means the exec session's TERM variable will default to "dumb". Users can override this by
setting TERM=xterm (or whatever is appropriate) to get the correct "smart" terminal behavior.
Currently attach and the editor do not share the same logic for saving
and restoring the terminal, and are not suitable for nesting (when the
caller wants to create something, attach, and then delete something when
the attach is over). This commit moves the interrupt protection logic
to a util package and supports nesting interrupt handlers.
