fix operator>=(bufferlist& l, bufferlist& r)

  bufferlist a;
  a.append("A");
  bufferlist ab;
  ab.append("AB");

a >= ab failed, throwing an instance of 'ceph::buffer::end_of_buffer'
because it tried to access a[1]. All comparison operators should be
tested using a lexicographic sort like strcmp or memcmp (-1, 0, 1).
In the meantime, the missing test is added:

  if (l.length() == p && r.length() > p) return false;

A set of unit tests demonstrating the problem and covering all comparison
operators are added to show that the proposed fix works as expected.

http://tracker.ceph.com/issues/4157 refs #4157

Signed-off-by: Loic Dachary <loic@dachary.org>