Clean up sasl warnings.

Clean up sasl warnings.

These were caused by assigning callback functions to the
sasl_callback_t.proc member.  The callback type doesn't list any
parameters, because parameters vary by callback.  The fix was simply
assigning a cast.

Cyrus-sasl2 has a sasl_callback_ft typedef that their sample code uses
for this purpose, but it is in a different header, saslplug.h, and
isn't in their 1.5 tree.  Since this is probably not portable to other
implementations, I just added an equivalent cast.