patch 8.1.1256: cannot navigate through errors relative to the cursor

Problem:    Cannot navigate through errors relative to the cursor.
Solution:   Add :cabove, :cbelow, :labove and :lbelow. (Yegappan Lakshmanan,
            closes #4316)

src/ex_docmd.c

@@ -56,6 +56,7 @@ static int	getargopt(exarg_T *eap);
 # define ex_cbuffer		ex_ni
 # define ex_cc			ex_ni
 # define ex_cnext		ex_ni
+# define ex_cbelow		ex_ni
 # define ex_cfile		ex_ni
 # define qf_list		ex_ni
 # define qf_age			ex_ni