Reopen debug.log on SIGHUP

The best log rotation method formerly available was to configure
logrotate with the copytruncate option.  As described in the logrotate
documentation, "there is a very small time slice between copying the
file and truncating it, so some logging data might be lost".

By sending SIGHUP to the server process, one can now reopen the debug
log file without losing any data.

src/init.cpp

@@ -80,6 +80,10 @@ void HandleSIGTERM(int)
     fRequestShutdown = true;
 }
 
+void HandleSIGHUP(int)
+{
+    fReopenDebugLog = true;
+}
 
 
 
@@ -285,7 +289,13 @@ bool AppInit2()
     sa.sa_flags = 0;
     sigaction(SIGTERM, &sa, NULL);
     sigaction(SIGINT, &sa, NULL);
-    sigaction(SIGHUP, &sa, NULL);
+
+    // Reopen debug.log on SIGHUP
+    struct sigaction sa_hup;
+    sa_hup.sa_handler = HandleSIGHUP;
+    sigemptyset(&sa_hup.sa_mask);
+    sa_hup.sa_flags = 0;
+    sigaction(SIGHUP, &sa_hup, NULL);
 #endif
 
     fTestNet = GetBoolArg("-testnet");