keyfile: add helper functions to record loaded UUID files

This code will be used later.

We want to remember which keyfiles are currently loaded (or hidden).

With the addition or multiple keyfile directories (soon), there are
two cases where this matters:

 - if there are multiple keyfiles which reference the same UUID,
   we can only load one of them. That is already a problem today
   with only one keyfile directory, where multiple files can reference
   the same UUID.
   The implementation will pick the file based on priorities (like
   the file modification date). However, the user may call explicitly
   call `nmcli connection load`. In that case, we cannot reload
   all files to find out whether the to be loaded file is hidden
   according to the defined priorities. We cannot do that, because we
   must not make decisions based on files on disk, which we are not told
   to reload. So, during a `nmcli connection load` we must look at
   unrelated files, to determine how to load the file.
   Instead, we do allow the user to load any file, even if it would be
   shadowed by other files. When we do that, we may want to persist which
   file is currently loaded, so that a service restart and a `nmcli connection
   reload` does not undo the load again. This can be later later be solved by
   writing a symlink

       "/var/run/NetworkManager/system-connections/.loaded-$UUID.nmkeyfile"

   which targets the currently active file.

 - if a profile was loaded from read-only persistant storage, the user
   may still delete the profile. We also need to remember the deletion
   of the file. That will be achieved by symlinking "/dev/null" as
   "/etc/NetworkManager/system-connections/.loaded-$UUID.nmkeyfile".

Add helper functions to read and write these symlinks.
5 jobs for th/keyfile-loaded-uuid in 18 minutes and 9 seconds (queued for 117 minutes and 32 seconds)
Status Job ID Name Coverage
  Test
passed #52846
checkpatch

00:01:14

passed #52847
f28_build_autotools

00:07:53

passed #52848
f28_build_meson

00:04:52

passed #52849
f28_rpm_autotools_dbg

00:07:10

passed #52850
f28_rpm_meson_dbg

00:13:01