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.h

@@ -266,7 +266,9 @@ public:
         // Make sure not to reuse time indexes to keep things in the same order
         int64 nNow = (GetTime() - 1) * 1000000;
         static int64 nLastTime;
-        nLastTime = nNow = std::max(nNow, ++nLastTime);
+        ++nLastTime;
+        nNow = std::max(nNow, nLastTime);
+        nLastTime = nNow;
 
         // Each retry is 2 minutes after the last
         nRequestTime = std::max(nRequestTime + 2 * 60 * 1000000, nNow);