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/os_amiga.c b/src/os_amiga.c
index 1e3fd4b..58e462d 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -1387,7 +1387,11 @@
     if ((mydir = CurrentDir(mydir)) != 0) // make sure we stay in the same directory
 	UnLock(mydir);
     if (tmode == TMODE_RAW)
+    {
+	// The shell may have messed with the mode, always set it.
+	cur_tmode = TMODE_UNKNOWN;
 	settmode(TMODE_RAW);		// set to raw mode
+    }
 #ifdef FEAT_TITLE
     resettitle();
 #endif