Break circular dependency: init -> * -> init by extracting shutdown.h

Most includers just wanted to react to pending shutdown.

This isolates access to `fRequestShutdown` and limits access to the shutdown
api functions, including the new `AbortShutdown` for setting it to `false`.

Note I originally called `AbortShutdown` `CancelShutdown` but that name was
already taken by winuser.h
https://travis-ci.org/bitcoin/bitcoin/jobs/386913329

This change also triggered a build error in bench. Fixing it required moving LIBBITCOIN_SERVER after LIBBITCOIN_WALLET in bench_bench_bitcoin_LDADD To make
server definitions in src/net.cpp available to wallet methods in
src/wallet/wallet.cpp. Specifically, solving:

  libbitcoin_wallet.a(libbitcoin_wallet_a-wallet.o): In function `CWalletTx::RelayWalletTransaction(CConnman*)':
  wallet.cpp:(.text+0x3f0e): undefined reference to `CConnman::NodeFullyConnected(CNode const*)'
  collect2: error: ld returned 1 exit status

https://travis-ci.org/bitcoin/bitcoin/jobs/392133581

Need for remaining init.h includes confirmed via a thorough search with a more
specific regex:
  \bInterrupt\(\)|\bShutdown\(\)|\bInitLogging\(\)|\bInitParameterInteraction\(\)|\bAppInitBasicSetup\(\)|\bAppInitParameterInteraction\(\)|\bAppInitSanityChecks\(\)|\bAppInitLockDataDirectory\(\)|\bAppInitMain\(\)|\bSetupServerArgs\(\)|\bLicenseInfo\(\)|g_wallet_init_interface|init.h

src/interfaces/node.cpp

@@ -21,6 +21,7 @@
 #include <primitives/block.h>
 #include <rpc/server.h>
 #include <scheduler.h>
+#include <shutdown.h>
 #include <sync.h>
 #include <txmempool.h>
 #include <ui_interface.h>