Implement chmod and fchmod for Unix targets

[el-ev]

Closes #4202

I was unable to write a test because FileMetadata only returns mode 0o100000 for the files, no permission information is provided.

[oli-obk]

Please also add tests for the new functions

[el-ev]

Please also add tests for the new functions

I've tried. Those tests would rely on file.metadata().unwrap().permissions() to check if the permissions are updated successfully. Which however doesn't return the correct permissions (only 0o100000 instead) under miri. Could you please provide some suggestions on this?

[oli-obk]

Maybe the shim we use to read permission is unfinished and doesn't work at all even though it could work on Unix?

[el-ev]

Yes, according to miri::shims::unix::fs::FileMetadata::from_meta, the perm field contains nothing more than the type of the file.