Compile with extra warnings turned on. And more makefile/code tidying up.

This turns on most gcc warnings, and removes some unused variables and other code that triggers warnings.
Exceptions are:
 -Wno-sign-compare : triggered by lots of comparisons of signed integer to foo.size(), which is unsigned.
 -Wno-char-subscripts : triggered by the convert-to-hex functions (I may fix this in a future commit).

src/net.cpp

@@ -938,7 +938,6 @@ void ThreadMapPort2(void* parg)
     char port[6];
     sprintf(port, "%d", GetListenPort());
 
-    const char * rootdescurl = 0;
     const char * multicastif = 0;
     const char * minissdpdpath = 0;
     struct UPNPDev * devlist = 0;
@@ -960,8 +959,6 @@ void ThreadMapPort2(void* parg)
     r = UPNP_GetValidIGD(devlist, &urls, &data, lanaddr, sizeof(lanaddr));
     if (r == 1)
     {
-        char intClient[16];
-        char intPort[6];
         string strDesc = "Bitcoin " + FormatFullVersion();
 #ifndef UPNPDISCOVER_SUCCESS
     /* miniupnpc 1.5 */