config.c: Add support for replacing variables names from strings

config.c: Add support for replacing variables names from strings

Read all unknown config options from the config file and store them in
the config variable array.

Implement a new function that can be used to replace variable names
from a string with the contents of the variable name in question.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>