link: Add --expire and --unlink flags

This adds expire and unlink fields to the PublicLink interface.

This fixes up the affected backends and removes unlink parameters
where they are present.

backend/mailru/mailru.go

@@ -1450,7 +1450,7 @@ func (f *Fs) DirMove(ctx context.Context, src fs.Fs, srcRemote, dstRemote string
 }
 
 // PublicLink generates a public link to the remote path (usually readable by anyone)
-func (f *Fs) PublicLink(ctx context.Context, remote string) (link string, err error) {
+func (f *Fs) PublicLink(ctx context.Context, remote string, expire fs.Duration, unlink bool) (link string, err error) {
 	// fs.Debugf(f, ">>> PublicLink %q", remote)
 
 	token, err := f.accessToken()