If a multivar exists within two sections (of the same name)
then they should both be updated in a `set_multivar`.  Ensure
that this is the case.

Add a test that exposes a bug in config_write.
It is valid to have multiple separate headers for the same config section, but
config_write will exit after finding the first matching section in certain
situations.
This test proves that config_write will duplicate a variable that already
exists instead of overwriting it if the variable is defined under a duplicate
section header.

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.

Instead of using a config file in resources, include the config file content to
be tested directly in the test.