Remove unsafety from unsupported/rwlosck.rs by using a Cell.
Remove unsafety from unsupported/rwlosck.rs by using a Cell.

Replacing the UnsafeCell by a Cell makes it all safe.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust