savedir: add streamsavedir, deprecate fdsavedir

savedir: add streamsavedir, deprecate fdsavedir

* NEWS: Mention deprecation of fdsavedir.
* lib/savedir.c (streamsavedir): New extern function, whose name
ends in "savedir" to be consistent with the others.  This differs
from savedirstream in that it doesn't close its argument.  The
next version of GNU tar will use this instead of fdsavedir, to
avoid some race conditions and conserve file descriptors.
(savedirstream): Reimplement as a wrapper around streamsavedir.
(fdsavedir): Add a comment deprecating this function.  As far as
I know, only GNU tar used it, and GNU tar doesn't need it any more.
* lib/savedir.h (streamsavedir): New decl.
(fdsavedir): Add a comment deprecating this.