Merge branch 'backup_crash_test' into 'master'

doc/man/knotc.8in

doc/man_knotc.rst

doc/operation.rst

src/knot/ctl/commands.c

src/knot/events/handlers/backup.c

src/knot/zone/backup.c

src/knot/zone/backup.h

src/libknot/errcode.h

src/libknot/error.c

tests-extra/tests/zone/backup_multi/test.py

tests-extra/tests/zone/backup_nosync/test.py