patch 8.1.2225: the "last used" info of a buffer is under used

Problem:    The "last used" info of a buffer is under used.
Solution:   Add "lastused" to getbufinfo(). List buffers sorted by last-used
            field. (Andi Massimino, closes #4722)
diff --git a/src/option.c b/src/option.c
index 3314a3c..1140a01 100644
--- a/src/option.c
+++ b/src/option.c
@@ -6987,6 +6987,8 @@
 	    new_wim_flags[idx] |= WIM_FULL;
 	else if (i == 4 && STRNCMP(p, "list", 4) == 0)
 	    new_wim_flags[idx] |= WIM_LIST;
+	else if (i == 8 && STRNCMP(p, "lastused", 8) == 0)
+	    new_wim_flags[idx] |= WIM_BUFLASTUSED;
 	else
 	    return FAIL;
 	p += i;