Use crate in imports in the openssl crate

This was mostly automated using `cargo fix --edition`, except imports of
openssl_sys (renamed to ffi) were converted to crate::ffi, which was
reverted.

The same thing is not done for the openssl-sys crate because this breaks
ctest (used in systest to verify the -sys crate), see
https://github.com/gnzlbg/ctest/issues/70