Add sysctl completion

[faho]

See #1375

I'm not sure about the deprecated/aliased options - if I include them, that makes the output longer for things that do the same, but not including them means people have to read the man page if they've gotten the invocation from somewhere else to find out what they mean.

[zanchey]

This doesn't work on OS X 😞 see #1745 for more.

[faho]

I just attempted to add support for OSX's weird implementation. The best way I could see was to just try sysctl -h, which OSX sysctl doesn't seem to support, so it should fail and allow us to use a completely different completion (as they are).

It also uses ":" as a key/value separator, so my assumption was that the output of sysctl -a looks like:

flounder:flat

Do note that I DID NOT TEST THIS as I don't have an OSX machine, I purely followed the comments and the manpage.

[ridiculousfish]

Looks good on OS X. Thanks for this! Merged as f359ded.