patch 9.1.0557: moving in the buffer list doesn't work as documented

Problem:  moving in the buffer list doesn't work as documented
          (SenileFelineS)
Solution: Skip non-help buffers, when run from normal buffers, else
          only move from help buffers to the next help buffer (LemonBoy)

As explained in the help section for :bnext and :bprev the commands
should jump from help buffers to help buffers (and from regular ones to
regular ones).

fixes: #4478
closes: #15198

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim.h b/src/vim.h
index a80f844..1b64e0d 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1074,6 +1074,8 @@
 // Values for flags argument of do_buffer()
 #define DOBUF_FORCEIT	1	// :cmd!
 #define DOBUF_NOPOPUP	2	// skip popup window buffers
+#define DOBUF_SKIPHELP	4	// skip or keep help buffers depending on b_help of the
+				// starting buffer
 
 // Values for sub_cmd and which_pat argument for search_regcomp()
 // Also used for which_pat argument for searchit()