Introduce home directory expansion function for config files, attribute files

Configuration backends have a readonly-flag which is currently used to
distinguish configuration snapshots. But somewhat unexpectedly, we do
not use the flag to prevent writing to a readonly backend but happily
proceed to do so.

This commit modifies logic to also honor the readonly flag for
configuration setters. We will now traverse through all backends and
pick the first one which is not marked as read-only whenever we want to
write new configuration.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.

This makes the API for commiting or discarding changes the same as for
references.

This lock/unlock pair allows for the cller to lock a configuration file
to avoid concurrent operations.

It also allows for a transactional approach to updating a configuration
file. If multiple updates must be made atomically, they can be done
while the config is locked.