Matching of plugins' debug streams was still case-sensitive. Also contains some
minor output tweaks.
It'd be nice to only list plugin debug streams actually _used_ by plugins. I
didn't see a quick way to do that so that's for another time.
Providing an unknown plugin debug stream with -B was previously silently
accepted. This caused user confusing as the behavior is "no output, but
seems to work".
Check the enabled debug streams once all plugins have been loaded and
exit early on for invalid streams.
$ ZEEK_PLUGIN_PATH=./build zeek -B plugin-zeek-myplugin -e 'print zeek_version();'
error in <command line>, line 3: No plugin debug stream 'plugin-zeek-myplugin' found
Closes#913.
