net/http: represent multi wildcards properly

net/http: represent multi wildcards properly

The routing tree used for matching ServeMux patterns used the
key "*" to hold a child node for a multi-segment wildcard.
The problem is that "*" is a valid path segment, which confused
the matching algorithm: it would fetch the multi wildcard child
when looking for the literal child for "*".

Eschew clever encodings. Use a separate field in the node to
represent the multi wildcard child.

Fixes #67067.

Change-Id: I300ca08b8628f5367626cf41979f6c238ed8c831
Reviewed-on: https://go-review.googlesource.com/c/go/+/582115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>