kmeta/ChildName() function needs more finer trimming.

[megumish]

https://github.com/knative/pkg/blob/1ad02a5662d29355fbd23d13cce08b2677ece901/kmeta/names.go#L60

Sometimes resource generation does not work without validation of . as well as -.

It may be a good idea to modify the terminating character if it is not an Alphanumeric character.

related to https://github.com/knative-sandbox/net-contour/issues/785