* configure.in: New configure option: --with-sha-crypt enabled by

* configure.in: New configure option: --with-sha-crypt enabled by
  default. Keeping the feature enabled is safe. Disabling it permits
  to disable the references to the SHA256 and SHA512 password
  encryption algorithms from the usage help and manuals (in addition
  to the support for these algorithms in the code).
* libmisc/obscure.c, libmisc/salt.c, src/newusers.c,
  src/chpasswd.c, src/chgpasswd.c, src/passwd.c: ENCRYPT_METHOD is
  always supported in login.defs. Remove the ENCRYPTMETHOD_SELECT
  preprocessor condition.
* libmisc/obscure.c, libmisc/salt.c, src/newusers.c,
  src/chpasswd.c, src/chgpasswd.c, src/passwd.c: Disable SHA256 and
  SHA512 if USE_SHA_CRYPT is not defined (this corresponds to a
  subset of the ENCRYPTMETHOD_SELECT sections).