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_unix.c b/src/os_unix.c
index aea3d5a..9162f5b 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4521,7 +4521,11 @@
     }
 
     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
@@ -4571,6 +4575,9 @@
     out_flush();
     if (options & SHELL_COOKED)
 	settmode(TMODE_COOK);		// set to normal mode
+    if (tmode == TMODE_RAW)
+	// The shell may have messed with the mode, always set it later.
+	cur_tmode = TMODE_UNKNOWN;
 
     if (unix_build_argv(cmd, &argv, &tofree1, &tofree2) == FAIL)
 	goto error;