Require libxcrypt-4.4.0 or newer and drop support of libcrypt

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.

README

@@ -212,7 +212,7 @@ REQUIREMENTS:
         newer though. TL;DR: turn audit off, still.
 
         glibc >= 2.31
-        libxcrypt or glibc (<= 2.38 built with --enable-crypt)
+        libxcrypt >= 4.4.0
         libmount >= 2.30 (from util-linux)
                 (util-linux *must* be built without --enable-libmount-support-mtab)
         libseccomp >= 2.4.0 (optional)