Bugfix: Make `backup` no longer hang on Solaris when seeing a FIFO file

The `backup` command used to hang on Solaris whenever it encountered a FIFO
file (named pipe), due to a bug in the handling of extended attributes. This
bug has now been fixed.

https://github.com/restic/restic/issues/4003
https://github.com/restic/restic/pull/4053