Rework configuration parsing ordering and overrides

Rework configuration parsing ordering and overrides

Currently, aside from list based settings, if a setting is used
multiple times across different configuration files, the later
assignments override earlier assignments. On top of that, the CLI
arguments are parsed last and override everything from config files.

This has worked very well until now, but breaks now that we have
[Match] sections. If we override a setting using the CLI, we want
any configured [Match] sections to compare against the value specified
via the CLI. Because the CLI values are applied last, this currently
isn't the case.

Similarly, if we add 90-local.conf to override the distribution release
used for Debian, all the config files that are processed earlier will
still compare against the default release configured for Debian in
50-debian.conf. If we rename 90-local.conf to 00-local.conf, the default
release in 50-debian.conf will still override that value. Only by putting
the override after the config file that assigns the default release but
before the first config file that matches on that release can we make this
work, but depending on the configuration this might require a lot of
different override files which isn't ideal.

Also, because later values can override earlier ones, it's possible to have
[Match] sections that match against different values of the same setting
both apply, if the setting happens to be reassigned inbetween, which is not
intuitive and error prone. Ideally, [Match] settings only apply to the final
value of a setting.

To fix these problems, this commit reworks the way we order and override
settings. Instead of having later assignments override earlier ones, for
all settings aside from list settings, the first assignment is the one that
is used. All later assignments of the same setting are ignored. Along with, we
switch to parsing CLI arguments first, which means that any settings assigned
in CLI args take precedence over those configured in config files.

Because the first value is immediately the final value, any [Match] sections
will only ever see the final value. One caveat is default values for settings.
We only want to assign these at the end of parsing, if no value has been
explicitly configured, but we also want [Match] sections to apply to default
values if no value is explicitly configured. The solution to this is that if
we encounter a setting in a [Match] section and it has not been explicitly
assigned a value yet, it is assigned its default value.

For list based settings, ! now configures an ignore glob, which means that if any
later assignments try to assign values that match an ignore glob, those values
are ignored. We also prepend list values instead of appending so that list
values that are configured in a preceeding config file appear later in the final
list value than values configured in a later assignment.

Implementation wise, this commit reworks config parser functions to return the
new value that should be assigned instead of assigning it themselves. This makes
the config parsing functions slightly more generic.