Mark c_vec functions as unsafe again
auto merge of #6339 : alexcrichton/rust/unsafe-cvec, r=catamorphism

As noted by @jwise [here](https://github.com/mozilla/rust/commit/52445129fdb4ee847332acbf516ced8f73b7990a#commitcomment-3172192), it's probably a good idea to keep these unsafe.

The lint check won't warn about these because it ignore `unsafe fn` declarations.