Bugfix: Do not crash with Synology NAS sftp server

It was found that when restic is used to store data on an sftp server on a
Synology NAS with a relative path (one which does not start with a slash), it
may go into an endless loop trying to create directories on the server. We've
fixed this bug by using a function in the sftp library instead of our own
implementation.

The bug was discovered because the Synology sftp server behaves erratic with
non-absolute path (e.g. `home/restic-repo`). This can be resolved by just using
an absolute path instead (`/home/restic-repo`). We've also added a paragraph in
the FAQ.

https://github.com/restic/restic/issues/2518
https://github.com/restic/restic/issues/2363
https://github.com/restic/restic/pull/2530