mount, cmount: add --attr-timeout to control attribute caching in kernel

This flag allows the attribute caching in the kernel to be controlled.
The default is 0s - no caching - which is recommended for filesystems
which can change outside the control of the kernel.

Previously this was at the default meaning it was 60s for mount and 1s
for cmount.  This showed strange effects when files changed on the
remote not via the kernel.  For instance Caddy would serve corrupted
files for a while when serving from an rclone mount when a file
changed on the remote.

cmd/mount/file.go

@@ -8,6 +8,7 @@ import (
 
 	"bazil.org/fuse"
 	fusefs "bazil.org/fuse/fs"
+	"github.com/ncw/rclone/cmd/mountlib"
 	"github.com/ncw/rclone/fs/log"
 	"github.com/ncw/rclone/vfs"
 	"golang.org/x/net/context"
@@ -24,6 +25,7 @@ var _ fusefs.Node = (*File)(nil)
 // Attr fills out the attributes for the file
 func (f *File) Attr(ctx context.Context, a *fuse.Attr) (err error) {
 	defer log.Trace(f, "")("a=%+v, err=%v", a, &err)
+	a.Valid = mountlib.AttrTimeout
 	modTime := f.File.ModTime()
 	Size := uint64(f.File.Size())
 	Blocks := (Size + 511) / 512