Rules versioning (#492)

* Add ability to print field names only

Add ability to print field names only instead of all information about
fields (description, etc) using -N cmdline option.

This will be used to add some versioning support steps that check for a
changed set of fields.

* Add an engine version that changes w/ filter flds

Add a method falco_engine::engine_version() that returns the current
engine version (e.g. set of supported fields, rules objects, operators,
etc.). It's defined in falco_engine_version.h, starts at 2 and should be
updated whenever a breaking change is made.

The most common reason for an engine change will be an update to the set
of filter fields. To make this easy to diagnose, add a build time check
that compares the sha256 output of "falco --list -N" against a value
that's embedded in falco_engine_version.h. A mismatch fails the build.

* Check engine version when loading rules

A rules file can now have a field "required_engine_version N". If
present, the number is compared to the falco engine version. If the
falco engine version is less, an error is thrown.

* Unit tests for engine versioning

Add a required version: 2 to one trace file to check the positive case
and add a new test that verifies that a too-new rules file won't be loaded.

* Rename falco test docker image

Rename sysdig/falco to falcosecurity/falco in unit tests.

* Don't pin falco_rules.yaml to an engine version

Currently, falco_rules.yaml is compatible with versions <= 0.13.1 other
than the required_engine_version object itself, so keep that line
commented out so users can use this rules file with older falco
versions.

We'll uncomment it with the first incompatible falco engine change.

userspace/engine/falco_engine_version.h

@@ -0,0 +1,27 @@
+/*
+Copyright (C) 2016-2018 Draios Inc dba Sysdig.
+
+This file is part of falco.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+*/
+
+// The version of rules/filter fields/etc supported by this falco
+// engine.
+#define FALCO_ENGINE_VERSION (2)
+
+// This is the result of running "falco --list -N | sha256sum" and
+// represents the fields supported by this version of falco. It's used
+// at build time to detect a changed set of fields.
+#define FALCO_FIELDS_CHECKSUM "32a91c003ab34f198dcb4c3100fbfb22bf402ad36549f193afa43d73f1f2eba3"