A new event bro_script_loaded() raised for each policy script loaded.

Also removing the -l command-line option as that can now be done at
the script-level.

A couple tests fail now that use -l. Leaving that until we have
script-level replacement.

testing/btest/policy/misc/loaded-scripts.bro

@@ -0,0 +1,7 @@
+# @TEST-EXEC: bro %INPUT >output
+# @TEST-EXEC: btest-diff output
+
+event bro_script_loaded(path: string, level: count)
+{
+    print level, path;
+}