This stops the cache cleaner running unnecessarily and saves
resources.
This also helps with issue #2227 which was caused by a second mount
deleting objects in the first mounts cache.
Before this change Open("name", os.O_RDONLY|os.O_TRUNC) would have
truncated the file. This is what Linux does, but is counterintuitive.
POSIX states this is undefined, so return an error in this case
instead. This preserves the invariant O_RDONLY => file is not
changed.
Background: cmd/mount/file.go Open() function does a Seek(0, 1) to see
if the file handle is seekable to set a FUSE hint. Before this change
the file was downloaded before it needed to be which was inefficient
(and broke beta.rclone.org because HEAD requests caused downloads!).
This adds new flags to mount, cmount, serve *
--cache-max-age duration Max age of objects in the cache. (default 1h0m0s)
--cache-mode string Cache mode off|minimal|writes|full (default "off")
--cache-poll-interval duration Interval to poll the cache for stale objects. (default 1m0s)
