If default-terminal is set to "screen" or "screen-*", emulate screen's

If default-terminal is set to "screen" or "screen-*", emulate screen's
historical (incorrect) behaviour for SGR 3 and send smso
(standout). Previously, we would send sitm (italics) if the terminal
outside had it and smso otherwise. This was acceptably until recently
because xterm's terminfo entry lacked sitm, so most users got smso.

People who want italics should set default-terminal to the forthcoming
"tmux" entry (and be prepared to deal with it being missing on older
hosts).

As a side-effect this changes default-terminal to be a server rather
than a session option.

suggested by and ok naddy