cfg-grammar: make it possible to create templates without braces

In a number of cases this syntax:

template foo {
    template("template text");
};

Has a lot of boilerplate. In case template-escape() option is not needed
(which is less-and-less needed and could probably be deprecated),
this syntax is just enough:

template foo "template text";

This plays nicely with the new feature that template functions can be defined in
configuration files with a similar syntax.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
plugin: add free_fn to Plugin instances

Traditionally Plugin instances are allocated as global variables, which
didn't need free operation. This patch allows allocating Plugin instances
dynamically, which in turn requires a free_fn.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
templates: add free_fn method to LogTemplateFunction

Just like with Plugins, LogTemplateFunctions couldn't really be subclassed,
as we basically assumed those are defined in a statically allocated struct.
With the upcoming support for user-defined template functions, we do need
to allocate these on the heap, thus a free operation is needed too.

This patch adds this free_fn, which should both free any dynamic data inside
the struct _and_ the struct itself.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
templates: don't allocate state if size_of_state is zero

The upcoming user template functions code does not have a state, and we
don't want to implement boilerplate free_state calls.  Make state optional
by specifying size_of_state as zero.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
templates: add implementation for user-defined template functions

This patch adds a new API, user_template_function_register() that allows
registering a LogTemplate instance as a template function, dynamically.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
cfg-grammar: add template-function support

This patch completes the support for user defined template functions. With
this patch you can define template functions in your configuration the same
way you would define a template.

template-function foobar "this is a template function running on $HOST";

destination d_file {
	file("something.log" template("$(foobar)\n"));
};

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
scl: add $(format-cim) template function

This patch adds a template function $(format-cim) into an SCL module
that implements Common Information Model (aka cim) formatting, as
used by Kibana (and Splunk).

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>