Send smkx/rmkx sequence at terminal init

Enter the "application" mode when setting up the terminal, this improves the
compatiblity with the standards; as a side effect now DEL key works ootb when
irssi is run in the suckless's st terminal.

src/fe-text/terminfo-core.h

@@ -88,6 +88,10 @@ struct _TERM_REC {
 
 	/* Beep */
         char *TI_bel;
+
+	/* Keyboard-transmit mode */
+	const char *TI_smkx;
+	const char *TI_rmkx;
 };
 
 extern TERM_REC *current_term;