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

tmux.1

@@ -2266,6 +2266,19 @@ Available server options are:
 Set the number of buffers; as new buffers are added to the top of the stack,
 old ones are removed from the bottom if necessary to maintain this maximum
 length.
+.It Ic default-terminal Ar terminal
+Set the default terminal for new windows created in this session - the
+default value of the
+.Ev TERM
+environment variable.
+For
+.Nm
+to work correctly, this
+.Em must
+be set to
+.Ql screen ,
+.Ql tmux
+or a derivative of them.
 .It Ic escape-time Ar time
 Set the time in milliseconds for which
 .Nm
@@ -2405,18 +2418,6 @@ or
 This option should be configured when
 .Nm
 is used as a login shell.
-.It Ic default-terminal Ar terminal
-Set the default terminal for new windows created in this session - the
-default value of the
-.Ev TERM
-environment variable.
-For
-.Nm
-to work correctly, this
-.Em must
-be set to
-.Ql screen
-or a derivative of it.
 .It Xo Ic destroy-unattached
 .Op Ic on | off
 .Xc