When we make runtime only changes, we may leave the profile in persistent
storage and store a overlay profile in /run. However, in various cases we need
to remember the original path.

Add code to store and retrieve that path from the keyfile.

Note that this data is written inside the keyfile in /run. That is because
this piece of information really depends on that particular keyfile, and not
on the profile/UUID. That is why we write it to the [.nmmeta] section of the
keyfile and not to the .nmmeta file (which is per-UUID).

This patch only adds the backend to write and load the setting from
disk. It's not yet used.