This change introduces NewRegisterSmartTransportWithOptions() to help
configure the smart transport with SmartSubtransportOptions. If the
default smart subtransport client needs to be configured, a newly
configured smart transport can be registered and used.
The SmartSubtransportOptions includes CABundle only for now.

This enables creating and using new transport with secrets that can be
deleted and not shared with subsequent operations.

The http client from httpSmartSubtransport is now shared with the
underlying httpSmartSubtransportStream, reusing the client and its
configurations.

It also fixes the error during cloning:
```
unable to clone: Post "http://test-user:***@127.0.0.1:40463/bar/test-reponame/git-upload-pack": io: read/write on closed pipe
```
by using credentials if available and avoiding failure due to
unauthorized request.

A user of the smart transport who needs to add a CA bundle in the
http client can do the following to setup the smart transport before
cloning:

```
stOpts := &git2go.SmartSubtransportOptions{CABundle: opts.CAFile}
rst, err := git2go.NewRegisterSmartTransportWithOptions("https", stOpts)
if err != nil {
	return err
}
if rst != nil {
	defer rst.Free()
}
```

Similar to SmartServiceActionUploadpack, SmartServiceActionReceivepack
is not an info endpoint. Fix the path for git-receive-pack.