Add support for reading config from a string.

I wanted to be able to set the log path and log size limit without having to create a temporary file, write these parameters (which are retrieved from non-file locations, this is a hard requirement in my use case) into the file, destroy the file.
Therefore, I made this new configFromString() API, extracting most of the code from parseConfig() to a new parseConfigLine() function, so that both config() and configFromString() can use it.

Signed-off-by: Lionel Debroux <lionel_debroux@yahoo.fr>