server/run: call block with an array of underlying servers

this lets the caller access low-level details (like resulting address
for a binding to port `0`)
bridge: make server root relative to .api path

This ensures that .apis behind a proxy work correctly.

For .apis behind a redirect where you want bridge requests to
go directly to the .api server, you can now specify a
`bridgeRoot` setting to @route.ExecutableDirectory which
will override the default relative path.