Running "go vet -shadow ./..." finds all places where a variable might
be incorrectly or unnecessarily shadowed. This fixes some of them.

Now instead of spawning a seperate thread we alternate between changing
the euid and ruid to both find the keyring and link it to the process
keyring. Note that we also ensure that the user keyring is linked into
the root keyring whenever possible.

This was creating an issue becasuse fully dropping privileges required
spawning a goroutine and using rutime.DropOSThread().

The functions are now changed to (Start|Stop)AsPamUser to indicate that
they handle privilege modification and keyring setup.