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/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index 6f5a73f..c09e234 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -1390,6 +1390,16 @@
     set verbose=0
 endfunc
 
+func Test_delfunction_force()
+    delfunc! Xtest
+    delfunc! Xtest
+    func Xtest()
+	echo 'nothing'
+    endfunc
+    delfunc! Xtest
+    delfunc! Xtest
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=4 tw=80 fdm=marker