If the terminal supports sitm for italics, use it instead of standout

(smso). From Tiago Resende.

tmux.h

@@ -323,6 +323,7 @@ enum tty_code_code {
 	TTYC_SMKX,	/* keypad_xmit, ks */
 	TTYC_SMSO,	/* enter_standout_mode, so */
 	TTYC_SMUL,	/* enter_underline_mode, us */
+	TTYC_SITM,	/* enter_italics_mode, it */
 	TTYC_VPA,	/* row_address, cv */
 	TTYC_XENL,	/* eat_newline_glitch, xn */
 };