Checkpointing the dynamic plugin code.

This is essentially the code from the dynamic-plugin branch except for
some pieces that I have split out into separate, earlier commits.

I'm going to updatre things in this branch going forward.

testing/btest/core/plugins/test-plugin.sh

@@ -0,0 +1,45 @@
+# @TEST-EXEC: ${DIST}/aux/bro-aux/plugin-support/init-plugin Demo Foo
+# @TEST-EXEC: bash %INPUT
+# @TEST-EXEC: make BRO=${DIST}
+# @TEST-EXEC: BROPLUGINS=`pwd` bro -NN | awk '/^Plugin:.*Demo/ {p=1; print; next} /^Plugin:/{p=0} p==1{print}' >>output
+# @TEST-EXEC: echo === >>output
+# @TEST-EXEC: BROPLUGINS=`pwd` bro -r $TRACES/empty.trace >>output
+# @TEST-EXEC: echo === >>output
+# @TEST-EXEC: BROPLUGINS=`pwd` bro demo/foo -r $TRACES/empty.trace >>output
+# @TEST-EXEC: btest-diff output
+
+cat >scripts/__load__.bro <<EOF
+@load ./demo/foo/base/at-startup.bro
+EOF
+
+cat >scripts/demo/foo/__load__.bro <<EOF
+@load ./manually.bro
+EOF
+
+cat >scripts/demo/foo/manually.bro <<EOF
+event bro_init()
+        {
+        print "plugin: manually loaded";
+        }
+EOF
+
+mkdir -p scripts/demo/foo/base/
+
+cat >scripts/demo/foo/base/at-startup.bro <<EOF
+event bro_init()
+        {
+        print "plugin: automatically loaded at startup";
+        print "calling bif", hello_plugin_world();
+        }
+EOF
+
+cat >src/functions.bif <<EOF
+function hello_plugin_world%(%): string
+        %{
+        return new StringVal("Hello from the plugin!");
+        %}
+EOF
+
+cat >src/events.bif <<EOF
+event plugin_event%(foo: count%);
+EOF