Switch stdout to line buffering

Use line buffering (instead of block buffering) so that messages arrive
immediately in systemd-journald, tail -f debug.log, and the like.

src/init.cpp

@@ -25,6 +25,7 @@
 #endif
 
 #include <stdint.h>
+#include <stdio.h>
 
 #ifndef WIN32
 #include <signal.h>
@@ -530,6 +531,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     fServer = GetBoolArg("-server", false);
     fPrintToConsole = GetBoolArg("-printtoconsole", false);
     fLogTimestamps = GetBoolArg("-logtimestamps", true);
+    setvbuf(stdout, NULL, _IOLBF, 0);
 #ifdef ENABLE_WALLET
     bool fDisableWallet = GetBoolArg("-disablewallet", false);
 #endif