Switching vectors from being 1-based to 0-based.

This is obviously a change that break backwards-compatibility. I hope
I caught all cases where vectors are used ...

I've completely removed the VECTOR_MIN constant. Turns out that was
already not working: some code pieces were nevertheless hard-coding
the 1-based indexing ...

testing/btest/istate/sync.bro

@@ -114,9 +114,9 @@ function modify()
 	
 	foo15 = 6667/tcp;
 	
-	foo16[4] = 4;
-	foo16[2] = 20;
-	++foo16[1];
+	foo16[3] = 4;
+	foo16[1] = 20;
+	++foo16[0];
 	
 	local x: type1;
 	x$a = "pop";

testing/btest/logging/vec.bro

@@ -18,8 +18,8 @@ event bro_init()
 
     local v: vector of string;
 
-	v[2] = "2";
-	v[5] = "5";
+	v[1] = "2";
+	v[4] = "5";
 
 	Log::write(SSH, [$vec=v]);
 }