libxcrypt: add

libxcrypt: add

libxcrypt is an external version of libc's libcrypt. It allows to use
algorithms now available with the libc. musl in OpenWrt for example
patches out several algorithms for size reasons. But for shadow-utils,
size does not really matter.

The hashes are set to solaris as that default gives a good balance
between compatibility and size. It includes:

bcrypt,
bcrypt_a,
sha512crypt,
sha256crypt,
md5crypt,
descrypt

The STRONG default adds several algorithms not supported by
shadow-utils.

Signed-off-by: Rosen Penev <rosenp@gmail.com>