Bugfix: Improve handling of interrupted syscalls in `mount` command

Accessing restic's FUSE mount could result in "input/output" errors when using
programs in which syscalls can be interrupted. This is for example the case for
Go programs. This has now been fixed by improved error handling of interrupted
syscalls.

https://github.com/restic/restic/issues/3567
https://github.com/restic/restic/issues/3694
https://github.com/restic/restic/pull/3875
