MINOR: spoe: Add support of negation for options in SPOE configuration file

For now, no options support negation (using "no" keyword). So it always returns
an error.
diff --git a/src/flt_spoe.c b/src/flt_spoe.c
index ddfb67b..950b6bc 100644
--- a/src/flt_spoe.c
+++ b/src/flt_spoe.c
@@ -3117,6 +3117,15 @@
                         err_code |= ERR_ALERT | ERR_FATAL;
                         goto out;
                 }
+
+		/* Following options does not support negation */
+		if (kwm == 1) {
+			Alert("parsing [%s:%d]: negation is not supported for option '%s'.\n",
+			      file, linenum, args[1]);
+			err_code |= ERR_ALERT | ERR_FATAL;
+			goto out;
+		}
+
 		if (!strcmp(args[1], "var-prefix")) {
 			char *tmp;