Faster timeout when connecting

Use non-blocking connects, and a select() call to wait a predefined
time (5s by default, but configurable with -timeout) for either
success or failure. This allows much more connections to be tried
per time unit.

Based on a patch by phantomcircuit.

src/init.cpp

@@ -151,6 +151,7 @@ bool AppInit2(int argc, char* argv[])
             "  -gen=0           \t\t  " + _("Don't generate coins\n") +
             "  -min             \t\t  " + _("Start minimized\n") +
             "  -datadir=<dir>   \t\t  " + _("Specify data directory\n") +
+            "  -timeout=<n>     \t  "   + _("Specify connection timeout (in milliseconds)\n") +
             "  -proxy=<ip:port> \t  "   + _("Connect through socks4 proxy\n") +
             "  -dns             \t  "   + _("Allow DNS lookups for addnode and connect\n") +
             "  -addnode=<ip>    \t  "   + _("Add a node to connect to\n") +
@@ -414,6 +415,13 @@ bool AppInit2(int argc, char* argv[])
         return false;
     }
 
+    if (mapArgs.count("-timeout"))
+    {
+        int nNewTimeout = GetArg("-timeout", 5000);
+        if (nNewTimeout > 0 && nNewTimeout < 600000)
+            nConnectTimeout = nNewTimeout;
+    }
+
     if (mapArgs.count("-printblock"))
     {
         string strMatch = mapArgs["-printblock"];