bitcoind: Daemonize using daemon(3)

Simplified version of #8278. Assumes that every OS that (a) is supported
by Bitcoin Core (b) supports daemonization has the `daemon()` function
in its C library.

- Removes the fallback path for operating systems that support
  daemonization but not `daemon()`. This prevents never-exercised code from
  ending up in the repository (see discussion here:
  https://github.com/bitcoin/bitcoin/pull/8278#issuecomment-242704745).

- Removes the windows-specific path. Windows doesn't support `daemon()`,
  so it don't support daemonization there, automatically.

Original code by Matthew King, adapted by Wladimir van der Laan.

src/bitcoind.cpp

@@ -9,6 +9,7 @@
 
 #include "chainparams.h"
 #include "clientversion.h"
+#include "compat.h"
 #include "rpc/server.h"
 #include "init.h"
 #include "noui.h"
@@ -127,29 +128,21 @@ bool AppInit(int argc, char* argv[])
             fprintf(stderr, "Error: There is no RPC client functionality in bitcoind anymore. Use the bitcoin-cli utility instead.\n");
             exit(1);
         }
-#ifndef WIN32
         if (GetBoolArg("-daemon", false))
         {
+#if HAVE_DECL_DAEMON
             fprintf(stdout, "Bitcoin server starting\n");
 
             // Daemonize
-            pid_t pid = fork();
-            if (pid < 0)
-            {
-                fprintf(stderr, "Error: fork() returned %d errno %d\n", pid, errno);
+            if (daemon(1, 0)) { // don't chdir (1), do close FDs (0)
+                fprintf(stderr, "Error: daemon() failed: %s\n", strerror(errno));
                 return false;
             }
-            if (pid > 0) // Parent process, pid is child process id
-            {
-                return true;
-            }
-            // Child process falls through to rest of initialization
-
-            pid_t sid = setsid();
-            if (sid < 0)
-                fprintf(stderr, "Error: setsid() returned %d errno %d\n", sid, errno);
+#else
+            fprintf(stderr, "Error: -daemon is not supported on this operating system\n");
+            return false;
+#endif // HAVE_DECL_DAEMON
         }
-#endif
         SoftSetBoolArg("-server", true);
 
         // Set this early so that parameter interactions go to console