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.

src/ex_cmds.h

@@ -426,7 +426,7 @@ EX(CMD_delcommand,	"delcommand",	ex_delcommand,
 			NEEDARG|WORD1|TRLBAR|CMDWIN,
 			ADDR_LINES),
 EX(CMD_delfunction,	"delfunction",	ex_delfunction,
-			NEEDARG|WORD1|CMDWIN,
+			BANG|NEEDARG|WORD1|CMDWIN,
 			ADDR_LINES),
 EX(CMD_display,		"display",	ex_display,
 			EXTRA|NOTRLCOM|TRLBAR|SBOXOK|CMDWIN,

src/testdir/test_vimscript.vim

@@ -1390,6 +1390,16 @@ 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

src/userfunc.c

@@ -2807,6 +2807,7 @@ ex_delfunction(exarg_T *eap)
     {
 	if (fp == NULL)
 	{
+	    if (!eap->forceit)
 		EMSG2(_(e_nofunc), eap->arg);
 	    return;
 	}

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    655,
 /**/
     654,
 /**/