plugin: Add InitPreExecution()

Currently, plugins do not have a way to further inspect or even mutate
script functions after ZAM optimization ran. One use-case here is
zeek-perf-support [1]. This plugin wraps Stmt instances of functions,
events and hooks hooks with a small assembly stub to support JIT map
files [2] and for integration with perf tools.

This change introduces a new InitPreExecution() hook that runs after
ZAM optimization completed, just before the zeek_init() event is enqueued.

Additionally, remove the existing CPP_activation_hook. It doesn't seem
to be used. If it becomes necessary in the future, the new
InitPreExecution() hook can be leveraged instead.

[1] https://github.com/zeek/zeek-perf-support
[2] https://github.com/torvalds/linux/blob/master/tools/perf/Documentation/jit-interface.txt

src/plugin/Manager.cc

@@ -484,6 +484,14 @@ void Manager::InitPostScript() {
         (*i)->InitPostScript();
 }
 
+void Manager::InitPreExecution() {
+    assert(init);
+
+    for ( plugin_list::iterator i = Manager::ActivePluginsInternal()->begin();
+          i != Manager::ActivePluginsInternal()->end(); i++ )
+        (*i)->InitPreExecution();
+}
+
 void Manager::FinishPlugins() {
     assert(init);