patch 8.2.4911: the mode #defines are not clearly named

Problem:    The mode #defines are not clearly named.
Solution:   Prepend MODE_.  Renumber them to put the mapped modes first.
diff --git a/src/os_unix.c b/src/os_unix.c
index d0675e8..a51f034 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4976,7 +4976,7 @@
 		p_more_save = p_more;
 		p_more = FALSE;
 		old_State = State;
-		State = EXTERNCMD;	// don't redraw at window resize
+		State = MODE_EXTERNCMD;	// don't redraw at window resize
 
 		if ((options & SHELL_WRITE) && toshell_fd >= 0)
 		{