pam_unix: Add support for crypt_checksalt, if libcrypt supports it.

pam_unix: Add support for crypt_checksalt, if libcrypt supports it.

libxcrypt v4.3 has added the crypt_checksalt function to whether
the prefix at the begining of a given hash string refers to a
supported hashing method.

Future revisions of this function will add support to check whether
the hashing method, the prefix refers to, was disabled or considered
deprecated by the system's factory presets or system administrator.
Furthermore it will be able to detect whether the parameters, which
are used by the corresponding hashing method, being encoded in the
hash string are not considered to be strong enough anymore.

*modules/pam_unix/passverify.c: Add support for crypt_checksalt.