patch 8.2.3725: cannot use a lambda for 'completefunc' and 'omnifunc'

Problem:    Cannot use a lambda for 'completefunc' and 'omnifunc'.
Solution:   Implement lambda support. (Yegappan Lakshmanan, closes #9257)
diff --git a/src/option.c b/src/option.c
index 94468d2..22dcfc5 100644
--- a/src/option.c
+++ b/src/option.c
@@ -5927,13 +5927,15 @@
 #ifdef FEAT_COMPL_FUNC
 	    buf->b_p_cfu = vim_strsave(p_cfu);
 	    COPY_OPT_SCTX(buf, BV_CFU);
+	    set_buflocal_cfu_callback(buf);
 	    buf->b_p_ofu = vim_strsave(p_ofu);
 	    COPY_OPT_SCTX(buf, BV_OFU);
+	    set_buflocal_ofu_callback(buf);
 #endif
 #ifdef FEAT_EVAL
 	    buf->b_p_tfu = vim_strsave(p_tfu);
 	    COPY_OPT_SCTX(buf, BV_TFU);
-	    buf_set_tfu_callback(buf);
+	    set_buflocal_tfu_callback(buf);
 #endif
 	    buf->b_p_sts = p_sts;
 	    COPY_OPT_SCTX(buf, BV_STS);