Forbid non-trivial paths in NotBefore

[robin-nitrokey]

NotBefore::Filename and NotBefore::FilenamePart store a PathBuf but the read_dir_first implementation only considers the file_name of this path. This can be confusing and lead to subtle bugs. I would suggest to either ensure that path.file_name() == path in the NotBefore constructors or to just use a raw string or byte array.

trussed/src/store/filestore.rs

Lines 171 to 178 in 805fe76

	.find(|(_, entry)| match not_before {
	NotBefore::None => true,
	NotBefore::Filename(path) => entry.file_name() == &**path,
	NotBefore::FilenamePart(path) => match entry.file_name().cmp_str(path) {
	Ordering::Less => false,
	Ordering::Equal | Ordering::Greater => true,
	},
	})