Write dbfiles atomically, renaming them once complete.
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.