switches: use switchId type from SDN_Types

switchId is a type synonym for VInt.t, and so is reverse compatible.
switches: Fix Empty regex's corresponding policy

Empty matches the empty string, not any string. It corresponds to drop,
not id.
switch: preserve link-free policy in Star case

When converting a policy to a regex, in the case of a Star, there is no
need to wrap the result in a Kleene if it is a PChar. In that case, the
Star can be lowered into the PChar.
switches: add comments and TODOs