fix: exit at-driver with error if run as admin

[mzgoddard]

When at-driver is run with admin privilege, clients of the named pipe will need the same privilege. It is easy to overlook this when starting at-driver and then using for example a screen reader that is not run with admin privilege. When the automation voice is loaded by said screen reader if it doesn't have equal or better privilege it will not be able to connect to the named pipe. At this time the voice cannot notify the user of this issue and appears to silently fail.

[jugglinmike]

This looks okay to me, but I think we'd both prefer a solution that doesn't involve constraining the end-user. At first, I thought there might be some way for the process to drop permissions, but I've since found a relatively new API that does the job.

I've opened gh-6 as an alternative to this patch.