When making new cache entries, write them to a file in a temporary

When making new cache entries, write them to a file in a temporary
directory first, and move the file to its permanent location only
after it has been fully written.  This prevents C-g from leaving
half-written cache entries.  (File folders should perhaps do this too,
but it is not clear where the temporary directory should be located,
and using /tmp or /var/tmp is not appropriate, since those directories
may reside on another file system, rendering rename(2) useless.)