unexpand: fix misalignment when spaces span a tabstop

unexpand: fix misalignment when spaces span a tabstop

The following dropped the space from the first field
  printf "1234567  \t1\n" | unexpand -a
Note POSIX says that spaces should not precede tabs.
Also a single trailing space should not be converted
if the next field starts with non blank characters.
So we enforce those rules too, with this change.

* src/unexpand.c (unexpand): Implement as per POSIX rules.
* tests/misc/unexpand: Add tests, and adjust existing
tests as per POSIX rules.
* NEWS: Mention the fix.
Reported by Hallvard B Furuseth