Catalog generation with groups

See merge request !1324

XDP-TCP: dont use structures if their allocation fails

See merge request !1327

dnssec: bugfix: dont retire key too soon after submission...

See merge request !1326

KSK remove is planned upon submission based on DS TTL
this overrided the planned remove with retire

backup improvements -- modified backup format (BACKUP_FORMAT_2), label file, improved backup locking, etc.

See merge request !1325

Returning KNOT_ENOTSUP makes more sense than returning KNOT_EMALF in such situations.

The test no longer works, as concurrent backups aren't possible in
Knot any more.

In case of repated server starts (enforced by occupied IP ports), an AXFR may happen
before the testing actually starts. Lower refresh value allows another AXFR of already
expired zone before the test time outs.

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.