- move snap folder to root folder, to allow automated builds
- install only polkit file outside of snap
- change path of bitmask-root accordingly in bitmask
- fix check for polkit file inside snap
- change and document the algorithm for picking bitmask-root in linux
- add LD_LIBRARY_PATH as an environment entry for bitmask-systray

The IS_${platform} constants are used in more places than the vpn
module. Let's define them in leap.bitmask.

we make a distinction between the system-wide bitmask-root, which should
be placed there by the maintainers of whatever packages your
distribution uses, and the bitmask-root that is placed by the bundles
(using polkit).

since the bundles copying over the helper from user-writeable folders
is a potential attack vector, we prefer to use the package's version if
present.

also, if we cannot find either, we abort the launching of the VPN. we've
discussed that this might move to the service initialization instead,
but I think the cases in which this is needed should be rare.

I fix also a corner-case in which we were using getcwd() at import time.
if you execute code and then remove the installation path, this will
raise a traceback in bitmaskctl. I think it's nicer to catch the error
properly when starting.