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.

d6abcd154c

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',
   },