config: examine whole file when writing

config: examine whole file when writing

Previously we would try to be clever when writing the configuration
file and try to stop parsing (and simply copy the rest of the old
file) when we either found the value we were trying to write,
or when we left the section that value was in, the assumption being
that there was no more work to do.

Regrettably, you can have another section with the same name later
in the file, and we must cope with that gracefully, thus we read the
whole file in order to write a new file.

Now, writing a file looks even more than reading.  Pull the config
parsing out into its own function that can be used by both reading
and writing the configuration.