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/ex_cmds.h b/src/ex_cmds.h
index 531bd0e..519cd08 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -426,7 +426,7 @@
 			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,