Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Caused by commit 0b4fa41a

Make attributes const, don't allow partial initialization.
Instantiate the CdioParanoiaInputStream object at the end of
input_cdio_open().

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

For strrchr().  Fixes build failure.

These are the default values already.

Replaces its own weird buffering code.

Code simplification.

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.