crypt(3) - Switch SHA256/512 to the Linux implementation

* The existing SHA backends have been shown to be more susceptible to brute-
force attacks than we would prefer --
http://www.openwall.co

crypt(3) - Switch SHA256/512 to the Linux implementation

* The existing SHA backends have been shown to be more susceptible to brute-
force attacks than we would prefer --
http://www.openwall.com/lists/oss-security/2012/01/16/2

* Bring in the reference implementation used in Linux, code is in the public
domain.

* Add required (standard) functions mempcpy and stpcpy.

* Change default for future installs to SHA512, this is the default on at
least archlinux and fedora.

* Add some minor hacks to libcrypt/crypt.c to ensure that
a) All existing passwords continue to work
b) All future passwords will be more secure with no changes required

* To update passwords to the new format use passwd(1) for each user, and to
change your default password type to SHA512 (default for new installations)
change the passwd_format setting under default to "sha512".

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