pam_unix: Add crypt_default method, if supported.

pam_unix: Add crypt_default method, if supported.

libxcrypt since v4.4.0 supports a default method for its
gensalt function on most system configurations.  As the
default method is to be considered the strongest available
hash method, it should be preferred over all other hash
methods supported by pam.

* modules/pam_unix/pam_unix.8.xml: Documentation for crypt_default.
* modules/pam_unix/passverify.c: Add crypt_default method.
* modules/pam_unix/support.h: Likewise.