about: complete other providers and re-work internals

* Implement about for:
    * local, crypt, cache, drive, swift, hubic, onedrive, pcloud, dropbox
  * Implement `--json` and `---full` flag for `rclone about`
  * change About interface to return a Usage structure
  * Remove operations.About as it is too thin an interface
  * Implement Integration test

Relates to #1138 and #1564

backend/local/about_unix.go

@@ -0,0 +1,29 @@
+// +build darwin dragonfly freebsd linux
+
+package local
+
+import (
+	"syscall"
+
+	"github.com/ncw/rclone/fs"
+	"github.com/pkg/errors"
+)
+
+// About gets quota information
+func (f *Fs) About() (*fs.Usage, error) {
+	var s syscall.Statfs_t
+	err := syscall.Statfs(f.root, &s)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to read disk usage")
+	}
+	bs := int64(s.Bsize)
+	usage := &fs.Usage{
+		Total: fs.NewUsageValue(bs * int64(s.Blocks)),         // quota of bytes that can be used
+		Used:  fs.NewUsageValue(bs * int64(s.Blocks-s.Bfree)), // bytes in use
+		Free:  fs.NewUsageValue(bs * int64(s.Bavail)),         // bytes which can be uploaded before reaching the quota
+	}
+	return usage, nil
+}
+
+// check interface
+var _ fs.Abouter = &Fs{}