MINOR: ncbuf: complete API and define block interal abstraction

MINOR: ncbuf: complete API and define block interal abstraction

Implement two new functions to report the total data stored accross the
whole buffer and the data stored at a specific offset until the next gap
or the buffer end.

To facilitate implementation of these new functions and also future
add/delete operations, a new abstraction is introduced : ncb_blk. This
structure represents a block of either data or gap in the buffer. It
simplifies operation when moving forward in the buffer. The first buffer
block can be retrieved via ncb_blk_first(buf). The block at a specific
offset is accessed via ncb_blk_find(buf, off).

This abstraction is purely used in functions but not stored in the ncbuf
structure per-se. This is necessary to keep the minimal memory
footprint.