join: add -o 'auto' to output a constant number of fields per line

join: add -o 'auto' to output a constant number of fields per line

Lines with a different number of fields than the first line,
will be truncated or padded.

* src/join.c (prfields): A new function refactored from prjoin(),
to output all but the join field.
(prjoin): Don't swap line1 and line2 when line1 is blank
so that the padding is applied to the right place.
(main): Handle the -o 'auto' option.
* tests/misc/join: Add 6 new cases to test the auto format.
* NEWS: Mention the change in behavior.
Suggestion from Assaf Gordon