Switch to mirage indenting style for server_unix.ml

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Avoid opening the event channel in a top-level binding

This is a privileged operation and one which is only needed if you
want to use the xen shared-memory interface. It's useful to run a
server unprivileged in userspace on a non-xen host for testing.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>