[build] Move policy settings to new src/policy/settings unit

This moves the following policy settings functions and globals to a new
src/policy/settings unit in lib_server:

- `incrementalRelayFee`
- `dustRelayFee`
- `nBytesPerSigOp`
- `fIsBareMultisigStd`

These settings are only required by the node and should not be accessed
by other libraries.

src/validation.cpp

@@ -22,6 +22,7 @@
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/rbf.h>
+#include <policy/settings.h>
 #include <pow.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
@@ -237,7 +238,6 @@ std::atomic_bool fImporting(false);
 std::atomic_bool fReindex(false);
 bool fHavePruned = false;
 bool fPruneMode = false;
-bool fIsBareMultisigStd = DEFAULT_PERMIT_BAREMULTISIG;
 bool fRequireStandard = true;
 bool fCheckBlockIndex = false;
 bool fCheckpointsEnabled = DEFAULT_CHECKPOINTS_ENABLED;