Register the consuming function and add a library context

The init function of the library now takes a function as argument to
allow a consumer using the library to control the function to be called
when data is ready in a buffer.

The kconsumerd_on_read_subbuffer_mmap and
kconsumerd_on_read_subbuffer_splice are now exported to allow a consumer
to use them directly if needed.

Also the library has now a context, where all local parameters are
registered instead of static variables. That way, we can have multiple
callers using the library within the same process. Only the flag
indicating that all fds are closed remain global to the library and
shared among callers.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>