src: add string preprocessor and use it for log prefix string

src: add string preprocessor and use it for log prefix string

Add a string preprocessor to identify and replace variables in a string.
Rework existing support to variables in log prefix strings to use it.

Fixes: e76bb3794018 ("src: allow for variables in the log prefix string")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>