crypt: Remove libcrypt support

All the crypt related functions, cryptographic algorithms, and
make requirements are removed,  with only the exception of md5
implementation which is moved to locale folder since it is
required by localedef for integrity protection (libc's
locale-reading code does not check these, but localedef does
generate them).

Besides thec code itself, both internal documentation and the
manual is also adjusted.  This allows to remove both --enable-crypt
and --enable-nss-crypt configure options.

Checked with a build for all affected ABIs.

Co-authored-by: Zack Weinberg <zack@owlfolio.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>