SMTP was improved to use the new abstract stuff a while ago,
but it was only implemented with raw socket abstract transport,
and a couple of 'api cheats' remained passing network information
for the peer connection through the supposedly abstract apis.
This patch adds a flexible generic token array to supply
abstract transport-specific information through the abstract apis,
removing the network information from the abstract connect() op.
The SMTP minimal example is modified to use this new method to
pass the network information.
The abstract transport struct was opaque, but there are real
uses to override it in user code, so this patch also makes it
part of the public abi.
