There’s no reason that “in” or “request” messages should be non-const.
This makes them const, bridges the const gap left by the MIG-generated
“check” functions with wrappers, and uses non-const fields in
“out” messages instead of const fields in “in” messages for in-out
parameters.
TEST=util_test ExcServerVariants.*:MachMessageServer.*
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/564533002