elevate privileges when calling clonepath

[trapexit]

The setting of euid and egid happens before any business logic to make sure all system calls run within the context of the caller (if mergerfs is running as root). When calling clonepath however we need to elevate those privileges back to root so we can properly create the destination path.

If it's not running as root then certain calls may fail depending on the permissions of the directories. Not much can be done there. One partial solution is to attempt to clone the path to another mount when an error occurs. The chance of inconsistency however still exists.