add a "checkproofs" make target to invoke coq's checkproofs
add a "checkproofs" make target to invoke coq's checkproofs

it allows specifying an arbitrary number of proof-checking workers, e.g.:

    make checkproofs J=32

it does not check universe constraints, so we should still use "make proof"
as a final end-to-end check.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl