Implement check in installer ensure admin rights, and warn about need for non-user writeable dir

[nhorman]

we need to ensure the following:

• That the windows installer is run in administrator context
• Warn user that user should install openssl in a non-user writeable directory

I think the approach here is pretty straight forward.

NSIS offers support to check if we are running as admin:
https://nsis.sourceforge.io/Check_if_the_current_user_is_an_Administrator

So we can add that in to the installer script

NSIS also supports the IsWriteable macro to check the permissions on a directory
https://nsis.sourceforge.io/IsWritable

so we should be able to check if the target directory is only system writeable

[mattcaswell]

Including checking the config file is not user writeable