Use socketpair to talk with the ipset command instead of pipe. For one,
we couldn't use send with pipe. For another, we are able to log its
output and see if something went wrong.

Use the new variadic functions for parsing & rendering of data, making
the code shorter in the process.

They allow to parse/render multiple elements at once, doing all the
necessary size checks.

Use it so the error messages are sent to the server.

Provide hooks and commands to create and update the sets.

• The hooks don't need the userdata parameter, that one is passed inside
  the storage parameter.
• Have a hook after complete replacement as well as before, to allow
  working on a copy and atomically swapping instead of just working or
  real data all the time.

• New module containing the set types and definitions.
• Each set gets its own function to convert its members (addresses) to
  strings that can be fed to the ipset utility.

Provide hooks for adding, removing and clearing of addresses.

If the set is present in both old and new config, migrate its content
and state, so we don't have to re-download everything.

Add a function to copy content of one address store into another.

Mark the import variable as possibly unused, suppressing a warning in
case the variable is created but not imported into and unused.

Be able to reload pluglibs after the plugin crashes and is reloaded.

This is to make sure the compiler can't prove the sig_handler never
exits and doesn't have the idea of legality of destroying the stack
below it. This is suspected reason why the backtraces from turris
routers are unusable.

Use the found filter, not the top-level one that might not be
differential.

It wasn't possible to load the library before, because the symbol
definition turned out to be static.

So it doesn't contain an underscore. It seems to trouble the buildroot.

No change in functionality, just take the diff_store functions and
structures and put them into a new pluglib. Link the pluglib to the flow
plugin.

Provide macros that can be used to list the functions only once, in
corresponding header file.