MINOR: buffer: add a few basic functions for the new API

MINOR: buffer: add a few basic functions for the new API

Here's the list of newly introduced functions :

- b_data(), returning the total amount of data in the buffer (currently i+o)

- b_orig(), returning the origin of the storage area, that is, the place of
  position 0.

- b_wrap(), pointer to wrapping point (currently data+size)

- b_size(), returning the size of the buffer

- b_room(), returning the amount of bytes left available

- b_full(), returning true if the buffer is full, otherwise false

- b_stop(), pointer to end of data mark (currently p+i), used to compute
  distances or a stop pointer for a loop.

- b_peek(), this one will help make the transition to the new buffer model.
  It returns a pointer to a position in the buffer known from an offest
  relative to the beginning of the data in the buffer. Thus, we can replace
  the following occurrences :

     bo_ptr(b)     => b_peek(b, 0);
     bo_end(b)     => b_peek(b, b->o);
     bi_ptr(b)     => b_peek(b, b->o);
     bi_end(b)     => b_peek(b, b->i + b->o);
     b_ptr(b, ofs) => b_peek(b, b->o + ofs);

- b_head(), pointer to the beginning of data (currently bo_ptr())

- b_tail(), pointer to first free place (currently bi_ptr())

- b_next() / b_next_ofs(), pointer to the next byte, taking wrapping
  into account.

- b_dist(), returning the distance between two pointers belonging to a buffer

- b_reset(), which resets the buffer

- b_space_wraps(), indicating if the free space wraps around the buffer

- b_almost_full(), indicating if 3/4 or more of the buffer are used

Some of these are provided with the unchecked variants using the "__"
prefix, or with the "_ofs" suffix indicating they return a relative
position to the buffer's origin instead of a pointer.

Cc: Olivier Houchard <ohouchard@haproxy.com>