Added support for renaming a datafile once it has been completely
written, and to write it using a temporary extension.

* src/server/ram-io.h (struct dbfile): Added fields fn,
tmp_extension and tmp_fn.  Added documentation.
(dbfile_open_write): New argument: tmp_extension.
(dbfile_rename): New function.
(dbfile_abort): New function.
* src/server/ram-io.c (dbfile_new): Initialize fn, tmp_extension
and tmp_fn.
(current_fn): New static function.  Return tmp_fn if non-NULL,
otherwise fn.
(dbfile_delete): Free the new fields fn, tmp_extension and
tmp_fn.
(dbfile_open_write): New argument: tmp_extension.  Store a copy of
the supplied filename.  If tmp_extension is non-NULL, store a copy
of it, and compute the actual file name to open as the filname
followed by the tmp_extension.
(fsync_dirname): New static function.
(dbfile_rename): New function.  Rename the file from tmp_fn to the
wanted final filename, and sync the directory.
(dbfile_abort): New function.
* src/server/dbck-cache.c (cache_sync_all): Pass a NULL pointer as
tmp_extension of dbfile_open_write(), to get the old behavior.
* src/server/simple-cache.c (pre_sync): Ditto.