truncate: improve handling of non regular files

truncate: improve handling of non regular files

Previously we copied `dd` and suppressed error messages
when truncating neither regular files or shared mem objects.
This was valid for `dd`, as truncation is ancillary to copying
it may also do, but for `truncate` we should display all errors.
Also we used the st_size from non regular files which is undefined,
so we display an error when the user tries this.

* src/truncate (do_truncate):  Error when referencing the size
of non regular files or non shared memory objects.  Display all
errors returned by ftruncate().
(main): Error when referencing the size of non regular files or
non shared memory objects.  Don't suppress error messages for
any file types that can't be opened for writing.
* tests/misc/truncate-dir-fail: Check that referencing the
size of a directory is not supported.
* tests/misc/truncate-fifo: Ensure the test doesn't hang
by using the `timeout` command.  Don't test the return from
running ftruncate on the fifo as it's system dependent as
to whether this fails or not.
NEWS: Mention the change in behavior.
Reported by Jim Meyering.