Merge lp://qastaging/~mir-team/mir/no-priv-headers-in-platforms-3 into lp://qastaging/mir

PASSED: Continuous integration, rev:3779
https://mir-jenkins.ubuntu.com/job/mir-ci/2030/
Executed test runs:
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-mir/2600
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-0-fetch/2663
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-1-sourcepkg/release=vivid+overlay/2655
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-1-sourcepkg/release=xenial+overlay/2655
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-1-sourcepkg/release=yakkety/2655
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=yakkety/2629
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=yakkety/2629/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=xenial+overlay/2629
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=xenial+overlay/2629/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=yakkety/2629
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=yakkety/2629/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=cross-armhf,compiler=gcc,platform=android,release=vivid+overlay/2629
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    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=android,release=vivid+overlay/2629
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=android,release=vivid+overlay/2629/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=mesa,release=xenial+overlay/2629
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=mesa,release=xenial+overlay/2629/artifact/output/*zip*/output.zip

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We certainly need to publish more to enable downstream platforms, but I'm not entirely convinced now is the right time to be doing this.

I won't block progress. But I wish we had achieved the original vision of a driver model that doesn't have two-way linkage between Mir and the driver.

And failing that, I also wish we had fewer headers and the ability to re-privatise some of them before so many get stuck in public.

It's also worth remembering that external drivers will be bound by the licenses we use in these headers. Presently that's LGPL, which is more appropriate than GPL, but still unacceptable for some proprietary vendors.

program_option.h: I've been thinking we could avoid exposing this dependency to drivers. Instead of drivers parsing command line options, users could provide options specific to a driver like:
  myserver --load mesa-kms.so:VT=1

Alternatively, we could audit all the driver-specific options and possibly find they're actually kind of common. So move them all to libmirserver and just optionally implement the behaviour in drivers as appropriate.