Continued revamping of libsmbclient.

Continued revamping of libsmbclient.

- James suggested using gcc's "deprecated" attribute to mark the context
  structure fields to generate warnings.  This creates a scenario with the
  best of all worlds.  I'm able to move to an organization that more easily
  allows future enhancements, while avoiding any mandatory changes by
  applications.  Thanks, James!

- Updated WHATSNEW.txt so that it accurately reflects the current state of
  affairs.

Derrell
(cherry picked from commit a67f96fbe9683b46c2149f7cb439d13f7f0e6ecd)
(This used to be commit 046653403fadfa2d42decce7048eca6321b321c6)