two additional slirp patches for emulators/qemu

Apply two additional slirp patches from the mailing list
(and quiet WITH_HACKS patching):

http://lists.gnu.org/archive/html/qemu-devel/2005-12/msg00277.html
(-> files/patch-slirp-nat)

http://lists.gnu.org/archive/html/qemu-devel/2005-05/msg00228.html
(-> files/patch-slirp-mbuf.c)

PR: ports/91358
Submitted by: Juergen Lock <nox@jelal.kn-bremen.de>