deprecate / replace `sml_transport` API

[r00t-]

the sml_transport API is far too primitive to be useful, sml_transport_read() does not have any provision to keep state between invocations, and no way to return more detailed information. (it can only return an int, and only write to stderr (see https://github.com/volkszaehler/libsml/issues/17 )

• we already re-defined the API so that a 0 return means "error": https://github.com/volkszaehler/libsml/pull/51
• we can not generate a warning (neither by returning to the caller, nor via stderr - because we can't even keep state) if the data read is being ignored when no start sequence is found (see e.g. https://github.com/volkszaehler/vzlogger/issues/633#issuecomment-1984188918 )