add support for binary sysctl payloads by handling them as hex

add support for binary sysctl payloads by handling them as hex
strings.

this was part of a demo showing how to implement the kernel side of
sysctl(3) for setting Semantically Opaque Interface Identifier key
material (for RFC 7217), but it seems to be the most straightforward
path toward integrating soiikey handling and rc.

Originally written by dlg, who commited it some time ago on my request.
I then backed it out again, now it's time to put it back in.

ok florian@ sthen@ naddy@ tb@

Man page bits tweaked & OK jmc