[mounts/sshfs] Try to take into consideration both ownership and permissions of a file.

**Describe the bug**
When doing a classic mount on a directory, we heavily restrict interaction with files which have uids/gids that are not mapped. This limits the interactions with a file even if its permissions should theoretically allow them i.e. a file with `0777` permissions should not care about ownership when trying to read from and write to it.

**To Reproduce**
How, and what happened?
1. Provide irrelevant mappings to override the default
```
multipass mount dir instance:dir -u 1234:1234 -g 1234:1234
```
2. Have a file with full permissions
```
sudo chmod 0777 dir/file
```
3. From inside the VM, the files still appears with full permissions
```
stat -c "%a" dir/file
777
```
4. But we can't use it
```
cat dir/file
cat: dir/file: Permission denied
```

**Expected behavior**
Take into consideration the permissions of files with unmapped uids/gids.

Start in the mounts/sshfs implementation and reproduce the issue with the supplied multipass mount, chmod, stat, and cat commands. Trace how unmapped uids/gids are handled and verify that access to files with permissive modes respects both ownership and permissions.

Written by the indexing model from the issue text.