Adds a new platform `unprivileged-qemu` which is meant to be ran without
root access. It has a restricted set of functionality, such as a lack of
the Local cluster and no networking between machines.
The new package is a near drop-in replacement for the standard os/exec
package. NewCommand has been renamed to Command in order to match the
standard package. dnsmasq has switched to this new package, fixing error
reporting in its Destroy method.
