snapuserd: Add a no-socket mode.

In first-stage init, during the selinux transition, no socket is needed.
It's even advantageous not to create one, since it greatly reduces the
amount of avc audits. This patch allows starting snapuserd with a preset
list of socket commands that it can run on startup.

Bug: 173476209
Test: manual test
Change-Id: I758d99097372e4dffb252e2836fd859b7fed162a