libbfd: Refine the API after Open Vswitch integration.

This commmit refines the libbfd API based on the integration to
Open Vswitch project.  Particularly, the following happens:
- Refine the comments.
- Define the auxiliary header file bfd-aux.h for per-thread variable,
  random number generation and logging.
  (bfd-aux_ovs.h is provided as reference)

Signed-off-by: Alex Wang <alexw@nicira.com>