patch 8.0.0655: not easy to make sure a function does not exist

Problem:    Not easy to make sure a function does not exist.
Solution:   Add ! as an optional argument to :delfunc.
diff --git a/src/userfunc.c b/src/userfunc.c
index de089bb..34da2ef 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2807,7 +2807,8 @@
     {
 	if (fp == NULL)
 	{
-	    EMSG2(_(e_nofunc), eap->arg);
+	    if (!eap->forceit)
+		EMSG2(_(e_nofunc), eap->arg);
 	    return;
 	}
 	if (fp->uf_calls > 0)