Add a function to query the underlying transport type. This allows the
dive computer backends to implement transport specific behaviour where
necessary.
For the built-in I/O implementations, the transport type is obviously
always hardcoded, but for a custom I/O implementation the application
needs to provide the correct type. Hence the transport type can't be
hardcoded in the vtable and needs to be passed as a parameter.
Wih the custom I/O implementation, an application can use its own
low-level I/O layer instead of using one of the built-in ones. The
application only needs to provide a set of callback functions, and
libdivecomputer will wrap them into a I/O stream.
