Windows: Update PATH for all users, not only the current one.

[cmarquardt]

The current Windows installer (optionally) updates the PATH environment variable for the user installing Docker-Toolbox.

In some environments, ordinary users don't have admin rights, but they still might have the option to use a dedicated 'local admin' account for installing additional software. When installing Docker-Toolbox in such an environment, the user's PATH environment variable is not updated; instead, the 'local admin' account receives the updated PATH. As a result, Docker Toolbox appears to be broken for the user when not being logged in as an administrator.

The problem is mentioned in (at least) #598, #621, #260, #484 and also #469. This PR fixes them by having the installer updating the system-wide rather than the user specific PATH. As before, users can choose to not modify the PATH at all.

Another workaround (in start.sh) was proposed in #482, but the latter only works if that shell script is started (and VirtualBox launched / local VMs generated / ...). Users using their own shell separate from running start.sh will continue to experience the problem.

Another symptom is that Kitematic doesn't find the docker installation for an ordinary user (mentioned in, e.g., #598).

On the other hand, #483 suggests to remove updating the PATH entirely because it doesn't have benefits (i.e., it does not work?). However, as updating the PATH is only an option, users can choose to keep the PATH variable as-is if they desire so and know what they are doing.