Initial revamp of the libsmbclient interface.

Initial revamp of the libsmbclient interface.

The libsmbclient interface has suffered from difficulty of improvement and
feature enrichment without causing ABI breakage.  Although there were a number
of issues, the primary ones were:

(a) the user of the library would manually manipulate the context structure
    members, meaning that nothing in the context structure could change other
    than adding stuff at the end;

(b) there were three methods of setting options: setting bits in a flags field
    within the context structure, setting explicit options variables within an
    options structure in the context structure, and by calling the
    smbc_option_set() function;

(c) the authentication callback did not traditionally provide enough
    information to the callee which required adding an option for a callback
    with a different signature, and now there are requests for even more
    information at the callback, requiring yet a third signature and option to
    set it (if we implement that feature).

This commit provides a reorganization of the code which fixes (a) and (b).
The context structure is now entirely opaque, and there are setter and getter
functions for manipulating it.  This makes maintaining ABI consistency much,
much easier.

Additionally, the options setting/getting has been unified into a single
mechanism using smbc_option_set() and smbc_option_get().

Yet to be completed is a refactoring of the authentication callback (c).

The test programs in examples/libsmbclient have been modified (if necessary;
some applications require no changes at all) for the new API and a few have
been minimally tested.

Derrell
(cherry picked from commit d4b4bae8ded824d06ad5ab0e219f71187ee5c771)