Storage/VD + Main/ExtPackManager+VirtualBox+SystemProperties: restructure previous change to fit better into the extpack design (using existing hooks) by using two new extpack helper functions

Main/ExtPackManager+Console: add a way to load HGCM modules from an extension pack, contributed by Jeff Westphal (partially rewritten). Thanks!

ExtPack: Changed the installed callback so that it can report back error info and trigger an immediate uninstall ifhoost incompatibilities are found.

ExtPack: Don't hold any locks when calling extension pack hooks (except for pfnUnload and pfnUninstall).

Main,Config.kmk,VBoxManage,ExtPacks: Moved the VRDE bits from IVirtualBox to the extension packs; changed ISystemProperties and IVRDEServer to talk about VRDE extension packs instead of VRDE libraries.

Main: Pass more IVirtualBox pointers to the extension pack. VRDE registration hook, VirtualBoxReady hook.

ExtPack: Fixes to IExtPack (QueryInterface can't be used for getting stuff from the ext pack, designed the plug-in interfaces). Bugfixes making 'VBoxManage list extpack' work when building with VBOX_WITH_EXTPACK_PUEL=1.

Main: More ExtPack code.

scm cleanup run.

ExtPack: More code.

fix OSE