drop support for old systems lacking vfork

drop support for old systems lacking vfork

Only really old systems (<=linux-2.0) lack a dedicated vfork system call.
The code that is in place to support them is causing issues with newer
arches that also don't provide a vfork system call -- instead, they do
vfork by calling clone in userspace.

If anyone cares about these really old systems, they can submit a patch
to make the system work with them while not breaking newer systems.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>