Update warning management

Update warning management

This change tidies up after the recent slew of warning reduction. It
updates README.WARNINGS to reflect the current state of the tree,
disables warnings for a couple of files that are now warning clean,
and ensures that the libuafs and libafsrpc Makefiles match the state
of the rest of the tree in terms of which warnings are enabled.

Reviewed-on: http://gerrit.openafs.org/672
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>