Make crypt(3) thread-safe by utilizing thread-local storage.

[delphij]

Summary

The crypt(3) interface is a wrapper around crypt_r(3) where a global
static storage is passed as data. Modernize the interface by
passing a thread-local storage instead so it is thread-safe.

Test Plan

kyua test lib/libcrypt

[jrtc27]

It's still not thread-safe if you pass the return value to another thread, but perhaps that's outside the scope