vim-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.

https://github.com/vim/vim/commit/d6abcd154cdc6a8dd4b7c6ccad37617ea8a1b4aa

src/nvim/eval.c

@@ -20863,7 +20863,9 @@ void ex_delfunction(exarg_T *eap)
 
   if (!eap->skip) {
     if (fp == NULL) {
-      EMSG2(_(e_nofunc), eap->arg);
+      if (!eap->forceit) {
+        EMSG2(_(e_nofunc), eap->arg);
+      }
       return;
     }
     if (fp->uf_calls > 0) {

src/nvim/ex_cmds.lua

@@ -692,7 +692,7 @@ return {
   },
   {
     command='delcommand',
-    flags=bit.bor(NEEDARG, WORD1, TRLBAR, CMDWIN),
+    flags=bit.bor(BANG, NEEDARG, WORD1, TRLBAR, CMDWIN),
     addr_type=ADDR_LINES,
     func='ex_delcommand',
   },

src/nvim/testdir/test_vimscript.vim

@@ -1242,6 +1242,16 @@ func Test_endfunction_trailing()
     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