Proof-checking code; fixups of segfaults and missing functionality in proof generation; fix bug 285.

* segfaults/assert-fails in proof-generation fixed, including bug 285
* added --check-proofs to automatically check proofs, like --check-models (but only for UF/SAT at present)
* proof generation now works in portfolio (but *not* --check-proofs, since LFSC code uses globals)
* proofs are *not* yet supported in incremental mode
* added --dump-proofs to dump out proofs, like --dump-models
* run_regression script now runs with --check-proofs where appropriate
* options scripts now support :link-smt for SMT options, like :link for command-line