zeek-setup: Support enabling debug streams through env variable

For debugging btests, it can be convenient to enable debug streams
by setting an environment variable rather than editing zeek invocations
and adding -B selectively.

Sample use case:

    $ export ZEEK_DEBUG_LOG_STREAMS=all
    $ btest -d core/failing-test.zeek
    $ less .tmp/core/failing-test/debug.log

This change makes Zeek's -B option and ZEEK_DEBUG_LOG_STREAMS are additive.

src/DebugLogger.h

@@ -85,6 +85,9 @@ public:
 
     bool IsEnabled(DebugStream stream) const { return streams[int(stream)].enabled; }
 
+    // Are any streams enabled?
+    bool HasEnabledStreams() const { return ! enabled_streams.empty(); }
+
     void SetVerbose(bool arg_verbose) { verbose = arg_verbose; }
     bool IsVerbose() const { return verbose; }