join: fix default check order behaviour

[cohosh]

If neither --nocheck-order or --check-order are specified, only fail on
unsorted inputs if either file contains unpaired lines.

To quote the relevant lines from info join:

   If the ‘--check-order’ option is given, unsorted inputs will cause a
fatal error message.  If the option ‘--nocheck-order’ is given, unsorted
inputs will never cause an error message.  If neither of these options
is given, wrongly sorted inputs are diagnosed only if an input file is
found to contain unpairable lines, and when both input files are non
empty.  If an input file is diagnosed as being unsorted, the ‘join’
command will exit with a nonzero status (and the output should not be
used).

This fixes a bunch of the remaining failing tests in #2634 including chkodr-4, chkodr-7, chkodr-5d, chkodr-5e, and for some reason 10b.