Bugfix: Ignore sockets for restore

We've received a report and correct the behavior in which the restore code
aborted restoring a directory when a socket was encountered. Unix domain socket
files cannot be restored (they are created on the fly once a process starts
listening). The error handling was corrected, and in addition we're now
ignoring sockets during restore.

https://github.com/restic/restic/issues/1730
https://github.com/restic/restic/pull/1731

