Add rules for libc structs

[lucic71]

This adds rules for both unsafe and recount libc structs. For unsafe the rule is trivial:

fn t1() -> libc::sockaddr {
    unsafe { std::mem::zeroed() }
}

For refcount, I added a new dir in libcc2rs, called libc_shims. It defines refcount shim structs for each libc struct, for example:

pub struct Sockaddr {
    pub sa_family: Value<u16>,
    pub sa_data: Value<Box<[u8]>>,
}

The shims exist for 2 reasons:

• generated code expects each field to be boxed
• libc fields can contain raw pointers, which are not permitted in refcount

The refcount rule for sockaddr becomes:

fn t1() -> libcc2rs::Sockaddr {
    Default::default()
}