MDEV-6137 better help for SET/ENUM sysvars

Auto-generate the allowed list of values for enum/set/flagset options
in --help output. But don't do that when the help text already has them.

Also, remove lists of values from help strings of various options, where
they were simply listed without any additional information.
archive: check frm version every time ARZ file is opened
archive: don't forget to set frmver_length=0 for old ARZ versions
better error message for TABLE_DEF_CHANGED and CRASHED_ON_USAGE in external_lock
archive: open ARZ files in external_lock, not in rnd_init/index_init