slirp: Remove obsolete backward-compatibility cruft

slirp: Remove obsolete backward-compatibility cruft

The slirp code does not use index() and gethostid() anymore,
so these parts can be removed without problems.
memmove() and strerror() should be available on each of the
supported platforms nowadays, too, so these wrappers are also
not needed anymore.
And we certainly also do not support Ultrix anymore, so no
need to keep the code for this platform anymore.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>