Make lsn_reset ("detach databases") optional and off by default.

Add an option -detachdb (and entry in OptionDialog), without which no
lsn_reset is called on addr.dat and blkindex.dat. That means these
files cannot be moved to a new environment, but shutdown can be
significantly faster. The wallet file is always lsn_reset'ed.

-detachdb corresponds to the old behaviour, though it is off by
default now to speed up shutdowns.

src/init.cpp

@@ -200,6 +200,7 @@ bool AppInit2(int argc, char* argv[])
 #else
             "  -upnp            \t  "   + _("Use Universal Plug and Play to map the listening port (default: 0)") + "\n" +
 #endif
+            "  -detachdb        \t  "   + _("Detach block and address databases. Increases shutdown time (default: 0)") + "\n" +
 #endif
             "  -paytxfee=<amt>  \t  "   + _("Fee per KB to add to transactions you send") + "\n" +
 #ifdef QT_GUI
@@ -255,6 +256,7 @@ bool AppInit2(int argc, char* argv[])
     }
 
     fDebug = GetBoolArg("-debug");
+    fDetachDB = GetBoolArg("-detachdb", false);
 
 #if !defined(WIN32) && !defined(QT_GUI)
     fDaemon = GetBoolArg("-daemon");