Move falco engine to its own library.

Move the c++ and lua code implementing falco engine/falco common to its
own directory userspace/engine. It's compiled as a static library
libfalco_engine.a, and has its own CMakeLists.txt so it can be included
by other projects.

The engine's CMakeLists.txt has a add_subdirectory for the falco rules
directory, so including the engine also builds the rules.

The variables you need to set to use the engine's CMakeLists.txt are:

- CMAKE_INSTALL_PREFIX: the root directory below which everything is
  installed.
- FALCO_ETC_DIR: where to install the rules file.
- FALCO_SHARE_DIR: where to install lua code, relative to the
- install/package root.
- LUAJIT_INCLUDE: where to find header files for lua.
- FALCO_SINSP_LIBRARY: the library containing sinsp code. It will be
- considered a dependency of the engine.
- LPEG_LIB/LYAML_LIB/LIBYAML_LIB: locations for third-party libraries.
- FALCO_COMPONENT: if set, will be included as a part of any install()
  commands.

Instead of specifying /usr/share/falco in config_falco_*.h.in, use
CMAKE_INSTALL_PREFIX and FALCO_SHARE_DIR.

The lua code for the engine has also moved, so the two lua source
directories (userspace/engine/lua and userspace/falco/lua) need to be
available separately via falco_common, so make it an argument to
falco_common::init.

As a part of making it easy to include in another project, also clean up
LPEG build/defs. Modify build-lpeg to add a PREFIX argument to allow for
object files/libraries being in an alternate location, and when building
lpeg, put object files in a build/ subdirectory.

userspace/engine/falco_common.h

@@ -0,0 +1,69 @@
+#pragma once
+
+#include <string>
+#include <exception>
+
+extern "C" {
+#include "lua.h"
+#include "lualib.h"
+#include "lauxlib.h"
+}
+
+#include <sinsp.h>
+
+//
+// Most falco_* classes can throw exceptions. Unless directly related
+// to low-level failures like inability to open file, etc, they will
+// be of this type.
+//
+
+struct falco_exception : std::exception
+{
+	falco_exception()
+	{
+	}
+
+	virtual ~falco_exception() throw()
+	{
+	}
+
+	falco_exception(std::string error_str)
+	{
+		m_error_str = error_str;
+	}
+
+	char const* what() const throw()
+	{
+		return m_error_str.c_str();
+	}
+
+	std::string m_error_str;
+};
+
+//
+// This is the base class of falco_engine/falco_output. It is
+// responsible for managing a lua state and associated inspector and
+// loading a single "main" lua file into that state.
+//
+
+class falco_common
+{
+public:
+	falco_common();
+	virtual ~falco_common();
+
+	void init(const char *lua_main_filename, const char *source_dir);
+
+	void set_inspector(sinsp *inspector);
+
+protected:
+	lua_State *m_ls;
+
+	sinsp *m_inspector;
+
+private:
+	void add_lua_path(std::string &path);
+};
+
+
+