join: with --check-order print offending file name, line number and data

* src/join (g_names): New global (was main's "names").
(main): Update all uses of "names".
(line_no[2]): New globals.
(get_line): Increment after reading each line.
(check_order): Print the standard "file name:line_no: " prefix
as well as the offending line when reporting disorder.
Here is a sample old/new comparison:
  -join: file 1 is not in sorted order
  +join: in:4: is not sorted: contents-of-line-4
* tests/misc/join: Change the two affected tests to expect
the new diagnostic.
Add new tests for more coverage: mismatch in file 2,
two diagnostics, zero-length out-of-order line.
* NEWS (Improvements): Mention it.
Suggested by David Gast in http://debbugs.gnu.org/9236

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