binpac: Add FlowBuffer policy mechanisms

This allows for tunability of the following behaviors:

* Minimum flowbuffer capacity to use when parsing a new unit

* Threshold at which flowbuffer capacity is contracted back to the
  minimum after parsing a complete unit and before parsing the next

* Maximum flowbuffer capacity to allow when parsing a given unit

Failed flowbuffer allocations due to reaching maximum capacity or any
other reason now throw ExceptionFlowBufferAlloc.

tools/binpac/lib/binpac_exception.h

@@ -116,6 +116,15 @@ public:
 		}
 };
 
+class ExceptionFlowBufferAlloc : public Exception
+{
+public:
+	ExceptionFlowBufferAlloc(const char* reason)
+		{
+		append(binpac_fmt("flowbuffer allocation failed: %s", reason));
+		}
+};
+
 }
 
 #endif  // binpac_exception_h