pppd: Use openssl for the DES instead of the libcrypt / glibc

pppd: Use openssl for the DES instead of the libcrypt / glibc

It seems the latest glibc (in Fedora glibc-2.27.9000-12.fc29) dropped
libcrypt.  The libxcrypt standalone package can be used instead, but
it dropped the old setkey/encrypt API which ppp uses for DES.  There
is support for using openssl in pppcrypt.c, but it contains typos
preventing it from compiling and seems to be written for an ancient
openssl version.

This updates the code to use current openssl.

[paulus@ozlabs.org - wrote the commit description, fixed comment in
 Makefile.linux.]

Signed-off-by: Jaroslav Škarvada <jskarvad@redhat.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>