bpo-28503: Use crypt_r() when available instead of crypt() (GH-11373) (GH-11376)

bpo-28503: Use crypt_r() when available instead of crypt() (GH-11373) (GH-11376)

Use crypt_r() when available instead of crypt() in the crypt module.

As a nice side effect: This also avoids a memory sanitizer flake as clang msan doesn't know about crypt's internal libc allocated buffer.
(cherry picked from commit 387512c7ecde6446f2e29408af2e16b9fc043807)

Co-authored-by: Gregory P. Smith <greg@krypto.org> [Google]