Commit b7221056 (slirp: clean up
conflicts with system headers) enclosed TCPOLEN_MAXSEG with an #ifdef
TCPOPT_EOL. This broke the build on illumos, which has TCPOPT_*
but not TCPOLEN_*.

Move them to their own #ifdef TCPOLEN_MAXSEG section to remedy this.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>