patch 8.2.0773: switching to raw mode every time ":" is used

Problem:    Switching to raw mode every time ":" is used.
Solution:   When executing a shell set cur_tmode to TMODE_UNKNOWN, so that the
            next time TMODE_RAW is used it is set, but not every time.
diff --git a/src/term.h b/src/term.h
index f1c3df5..b9535ee 100644
--- a/src/term.h
+++ b/src/term.h
@@ -209,6 +209,7 @@
 #define T_SSI	(TERM_STR(KS_SSI))	// save icon text
 #define T_SRI	(TERM_STR(KS_SRI))	// restore icon text
 
-#define TMODE_COOK  0	// terminal mode for external cmds and Ex mode
-#define TMODE_SLEEP 1	// terminal mode for sleeping (cooked but no echo)
-#define TMODE_RAW   2	// terminal mode for Normal and Insert mode
+#define TMODE_COOK	0   // terminal mode for external cmds and Ex mode
+#define TMODE_SLEEP	1   // terminal mode for sleeping (cooked but no echo)
+#define TMODE_RAW	2   // terminal mode for Normal and Insert mode
+#define TMODE_UNKNOWN   9   // after executing a shell