Clean up some remains from the gcc2 days.

Clean up some remains from the gcc2 days.

In gcc2, we kept __FreeBSD__ as a compiler built-in. Starting with gcc3,
we only define __DragonFly__.

* Remove the general definition of __FreeBSD_version from <sys/param.h>
  and <osreldate.h>. However, ipftest(1) and ipsend(1) still need it, so
  define it from their Makefiles.

* Remove the definition of __FreeBSD__ from the Makefiles of ipmon(8)
  and openssl(1), both of which don't need this anymore.