cuprated: add private file permissions for Windows

[redsh4de]

What

Closes #551 by adding the Windows equivalent.

Why

In cases where the data paths were outside of the current user's profile directory, files in cuprate's data directories could be read by other users on the same Windows machine.

Where

helper

• fs/windows_perms.rs - Looks up the current user's ID, builds the ACL, makes it the default for files cuprate creates, and makes each folder it creates with it.
• fs.rs - set_private_global_file_permisisons is now Unix-only (Windows doesn't really have a 1-to-1 global equivalent). A new Windows-only set_private_directory_permissions(roots: &[&Path]) passes the folder paths to windows_perms::apply. Each function only exists on the OS that needs it
• Cargo.toml - added new create features from windows that are needed for this to work

cuprated

• config.rs - new Config::writable_directories() returns the directories cuprate writes to. On Windows only, read_config_and_args calls set_private_directory_permissions once the paths are resolved. This also gave a opportunity to rewrite check_dir_permissions to loop over the new helper.
• main.rs - set_private_global_file_permissions call is now Unix-only

How

The SDDL string is O:{user}D:P(A;OICI;FA;;;{user})(A;OICI;FA;;;SY)(A;OICI;FA;;;BA) (syntax reference):

Part	Meaning
O:{user}	Owner is the user
D:P	Start of the access list, with the "protected" flag so it does not inherit from the parent folder
(A;OICI;FA;;;{user})	Allow the user full access; sub-folders and files inherit it
(A;OICI;FA;;;SY)	Same for SYSTEM
(A;OICI;FA;;;BA)	Same for BUILTIN\Administrators

Precedent: Win32-OpenSSH

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Example with a path like D:\Cuprate:

[SyntheticBird45]

I'll review the windows API usage more in details tomorrow. Nothing else to comment outside of it.