speedo,w32: Fix InstallDir usage and x64 install

* build-aux/speedo/w32/inst.nsi (InstallDir): Move to the
MUI definitions for clarity.
(MULTIUSER_INSTALLMODE_INSTDIR): Use pretty name.
(-gnupginst): Handle regviews and upgrades

As MultiUser.nsh sets the INSTDIR variable we have
to define MULTIUSER_USE_PROGRAMFILES64.

The resulting installer created by makensis itself
is still a 32bit binary we have to use SetRegView to
use the correct registry hive for 64 bit software.

To better handle the path switch from ProgramFiles (x86)
to ProgramFiles on update we now call the uninstaller
of the previous version before installing the new
version. This additionally ensures that there
are no leftover files and a dirty install directory
after some upgrades.