Some changes to the function interfaces.

Also, make the warning message more correct.

It's enough to count planned backup events. Counting finished zones isn't necessary.

The new string can be used in other situations better in the future.
The original string hasn't appeared in a release yet.

It's a little bit more code and one additional stat() call, but it's more resilient to
future changes in the code. From user's point of view the functionality is the same.

It fixes error reporting from syscalls and functions such as make_dir().

Return KNOT_EEXIST from make_dir() when the target already exists, but it isn't a directory.
The previous error code could be misleading and it hadn't been defined in the first place.

Functional changes:

1. Zone files are stored in a dedicated directory.
2. Zone files use the zones' default file names in the backup,
   not the file names defined in the configuration as before. This
   change eleminates file name conflicts which were possible with the
   original backup design.

Iff there isn't a label file and the force flag is set, use the original backup
format for restore.

Otherwise report a backup without a label file as malformed.

If there is an empty label file, the label file doesn't start with a label file
header, the label file doesn't include a backup format description, or the backup
format code isn't known, consider it a malformed backup and don't restore from it,
even if the force flag has been set.

Sort the #include statements alphabetically and remove a duplicity.
Break up a too long macro line.

The label file is a warrant that the backup has benn successfully completed
and can be safely used for a restore.

The "-f" parameter for zone-restore allows restore from older backups
which have been created without a backup label file.

The label file identifies the backup an prevents another backup to be made
to an already existing backup. Existence of a label file is necessary for a restore,
which helps to prevent a frequent user error and to avoid a number of failures
which would be triggered by it.

In addition to that, every label file contains a brief description
of the backup and its parameters. A list of zones that the backup contains
isn't included so far.

Basic implementation of TCP over XDP for Knot  server

See merge request !1300