Fix use of AuthDataWithContext capability

Fix use of AuthDataWithContext capability

During my initial plans for, and the subsequent discussion of a more
significant change to the API for libsmbclient, I had removed the
AuthDataWithContext usage, in favor of a more generalized planned interface.
When the API returned to its original state, I neglected to reinsert this
code.

Use of an authentication function with the context can be tested using

  examples/libsmbclient/testbrowse -C

Derrell
(This used to be commit 38eab68dfb2d8abe8ad00f5a86fc54c778d0d303)