Move POLICY_SERVER and sys.policy_hub initialization to single place

Previously it was scattered to several places, partially redundant.

Note that new clause in SetPolicyServer changes the behaviour, but
1) it is very rare case (policy_server.dat disappears during runtime)
2) this behaviour makes more sense: previously if policy_server.dat
disappeared, both POLICY_SERVER and sys.policy_hub retained old value
until process restart. After the change POLICY_SERVER and sys.policy_hub
pick up new value, even if it is "no file exists anymore"
Fix inclusion guards after rename
Remove unused variable
constify Policy