cp,mv,install: avoid opening non directory destination

cp,mv,install: avoid opening non directory destination

commit v9.0-66-ge2daa8f79 introduced an issue, for example
where cp could hang when overwriting a destination fifo,
when it would try to open() the fifo on systems
like Solaris 10 that didn't support the O_DIRECTORY flag.

This is still racy on such systems, but only in the
case where a directory is replaced by a fifo in
the small window between stat() and open().

* src/system.h (target_directory_operand): On systems without
O_DIRECTORY, ensure the file is a directory before attempting to open().
* tests/cp/special-f.sh: Protect cp with timeout(1),
as cp was seen to hang when trying to overwrite an existing fifo.
* NEWS: Mention the bug fix.