krb_udp.c warning fix

krb_udp.c warning fix

This file generates a warning because the left side of a variable
assignment is commented out.  Keep the effect of the line
(incrementing packet) but remove the unused casting and
reference, and remove the comments that date from the original
IBM source.
Leave a new comment in place in case the information is useful.

Adjust the Makefile and README.WARNINGS to account for the change.

Change-Id: I944e1c488e39411d32d700cba2d3ef567eddddb4
Reviewed-on: http://gerrit.openafs.org/804
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>