libcrypt was no longer built by default since glibc-2.38, and it has been
completely removed since glibc-2.39.
Let's always use libxcrypt, unless when building with musl. As already
major distribution already have libxcrypt-4.4.x, hence let's also bump
the required minimum version to 4.4.0.
libxcrypt cannot be built with musl, hence the previous fallback logic
in libcrypt-util.c are moved to musl/crypt.c.
Note, libxcrypt-4.4.0 was released on 2018-11-20.
See also #38608.
Yu Watanabe
5863641bb7