I/O functions with output parameters, should always initialize those
output parameters, even when an error is returned. This prevents the
(accidental) use of uninitialized variables, whenever the caller forgets
to check the return code.
As a nice side effect, the use of a local variable guarantees that the
underlying I/O implementation will always receive a valid pointer.
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.
The purpose of the new I/O interface is to provide a common interface
for all existing I/O implementations (serial, IrDA, bluetooth and USB
HID). With a common interface the dive computer backends can more easily
use different I/O implementations at runtime, without needing
significant code changes. For example bluetooth enabled devices can
easily switch between native bluetooth communication and serial port
emulation mode.
The new interface is modelled after the existing serial communication
api. Implementations where some of those functions are meaningless (e.g.
IrDA, bluetooth and USB), can just leave those functions unimplemented
(causing the call to fail with DC_STATUS_UNSUPPORTED), or implement it
as a no-op (always return DC_STATUS_SUCCESS).
