Always use relative paths in Haddock documentation

[liyang]

Is it not possible to always use relative instead of absolute file:// URLs? (Or at least make a best-effort for Windows.) This was mentioned in #509, #1064, #1406, and committed in 544d737 to cater for Windows platforms where C:\… is mistakenly interpreted as a protocol by some browsers.

As things currently stand, I cannot serve up my main Haddocks index for use when I'm ssh/VPNing into my dev machine from home. (Nor for other people in the office to use, for that matter.)

There's also a seconday issue in that Chrome forbids cookies for file:// URLs, such that haddock-util.js can't keep track of which sections were collapsed: untested workaround; related Chrome issue. Perhaps HTML5 local storage is the way forward there. So far I've been using a local HTTP server even when browsing on the machine in question.

[dcoutts]

In general this would rely on using relative package installs, which we do not yet have. Supporting relocatable installs would of course be a good thing.

[ttuegel]

See #462.