Add support for SHA256 and SHA512 encrypt methods. Apply RedHat's patch

Add support for SHA256 and SHA512 encrypt methods. Apply RedHat's patch
shadow-4.0.18.1-sha256.patch. Thanks to Peter Vrabec. Hardly no changes
except re-indent and changes related to recent modifications (max_salt_len
in crypt_make_salt). Changes in lib/defines.h not applied (definition of
ENCRYPTMETHOD_SELECT). I will add a configure check or flag.