Memory unsafety: data races in darwin.c

[tmfink]

I've found the following data races in darwin.c:

1. The static unsigned long long size is a mutable global that is read/modified without any synchronization.
   • https://github.com/FillZpp/sys-info-rs/blob/1c119b37ac0737466cd450f1fa7fb03af28024c8/c/darwin.c#L109
   • It looks like this is meant to be an optimization but it is unsound since it is called directly from the public Rust function mem_info(). Multiple Rust threads could racily write to the global variable size.
2. static int skipvfs is mutable global that is racily initizialized in makevfslist().
   • https://github.com/FillZpp/sys-info-rs/blob/1c119b37ac0737466cd450f1fa7fb03af28024c8/c/darwin.c#L20
   • disk_info() is a public Rust function that can be directly called from safe Rust code