Annotate intentional writes of const-qualified data (CID #1508484, #1517674) (#4872)

Annotate intentional writes of const-qualified data (CID #1508484, #1517674) (#4872)

Certain members of fr_value_box_t and hence of fr_pair_t have
const-qualified type, so pair_init_from_da() and fr_value_box_init(),
which modify some of those members, coverity points them out.
(UNCONST() placates the compiler, but not coverity.)

The coverity primitives that one might think would apply here,
__coverity_write_buffer_{bytes, elements}__(), are said to relate
mostly to checkers unrelated to writes to const-qualified fields,
so we annotate.