This lets us make a lot of other stuff const, allows the compiler to
generate (slightly) better code, and will make me get slightly fewer
patches from folks who stick mutable stuff into or_options_t.
const: because not every input is an output!
This is needed for IOCP, since telling the IOCP backend about all
your CPUs is a good idea. It'll also come in handy with asn's
multithreaded crypto stuff, and for people who run servers without
reading the manual.
