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.

We have corrected the error handling for interrupted syscalls.

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