snapuserd: Refactor daemon/server.

This is in preparation for moving to a traditional client/server model
where clients stay connected and the server multiplexes multiple
connections.

Client has been renamed to DmUserClient to differentiate it from local
socket clients.

poll() responsibilities have been moved into SnapuserdServer. In
addition, the server now tracks all open clients and polls them
together with the listen socket.

SnapuserDaemon is now only responsible for signal masking. These two
classes can probably be merged together - I didn't do that here because
the patch was already large.

Bug: 168554689
Test: manual test
Change-Id: Ibc06f6287d49e832a8e25dd936ec07747a1b0555