[BUG 5580] Allow access to DFS shares via libsmbclient

[BUG 5580] Allow access to DFS shares via libsmbclient

Brian Sheehan provided a nice patch intended for the 3.0 code base.  This
commit applies a similar patch for the 3.3 code base.  It adds a new public
function to libsmbclient -- smbc_set_credentials() -- that may be called from
the authentication callback when DFS referrals are in use.

Derrell