[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/Makefile.am

@@ -164,6 +164,7 @@ BITCOIN_CORE_H = \
   policy/fees.h \
   policy/policy.h \
   policy/rbf.h \
+  policy/settings.h \
   pow.h \
   protocol.h \
   psbt.h \
@@ -269,8 +270,8 @@ libbitcoin_server_a_SOURCES = \
   noui.cpp \
   outputtype.cpp \
   policy/fees.cpp \
-  policy/policy.cpp \
   policy/rbf.cpp \
+  policy/settings.cpp \
   pow.cpp \
   rest.cpp \
   rpc/blockchain.cpp \
@@ -436,6 +437,7 @@ libbitcoin_common_a_SOURCES = \
   netaddress.cpp \
   netbase.cpp \
   policy/feerate.cpp \
+  policy/policy.cpp \
   protocol.cpp \
   psbt.cpp \
   scheduler.cpp \