dd conv=unblock: print final newline consistently

dd conv=unblock: print final newline consistently

* src/dd.c (dd_copy) [C_UNBLOCK]: Always print the final newline for
non-empty output, not just when output size is a multiple of cbs.
* doc/coreutils.texi (dd invocation) [conv=unblock]: Mention that dd
prints a newline after each output record, not just when replacing
trailing spaces.
Reported by Ulrich Drepper.
* tests/dd/unblock: New file.  Test for this.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.