pseudofile: new destination driver to deliver messages to pseudo files

This is a very simple destination driver that can be used to deliver
messages to special files, like the ones in /proc or /dev.

It is much simpler than the file driver:
  - opens and closes the file for each message instead of keeping it opened
  - doesn't support templates in the name of the file
  - works in the context of the input driver, doesn't have an actual queue

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