Make the use of perl more consistent

- In Configure, register the perl interpreter used to run Configure,
  so that's the one being used throughout instead of something else
  that Configure happens to find.  This is helpful for using a perl
  version that's not necessarely first in $PATH:

    /opt/perl/5.22.1/bin/perl ./Configure

- Make apps/tsget a generated file, just like apps/CA.pl, so the
  perl interpreter registered by Configure becomes the hashbang path
  instead of a hardcoded /usr/bin/perl

Reviewed-by: Andy Polyakov <appro@openssl.org>