enum: Return boolean result for enum_from_name() lookup

Handling the result for enum_from_name() is difficult, as checking for
negative return values requires a cast if the enum type is unsigned. The new
signature clearly differentiates lookup result from lookup value.

Further, this actually allows to convert real -1 enum values, which could not
be distinguished from "not-found" and the -1 return value.

This also fixes several clang warnings where enums are unsigned.

src/conftest/hooks/unencrypted_notify.c

@@ -68,8 +68,7 @@ METHOD(listener_t, ike_updown, bool,
 		type = atoi(this->type);
 		if (!type)
 		{
-			type = enum_from_name(notify_type_names, this->type);
-			if (type == -1)
+			if (!enum_from_name(notify_type_names, this->type, &type))
 			{
 				DBG1(DBG_CFG, "unknown notify: '%s', skipped", this->type);
 				return TRUE;