Back out a bug I previously introduced by removing an "eval" in front

Back out a bug I previously introduced by removing an "eval" in front
of exec in proc nexec.

XORP can now be selected as a router model even if xorp is not installed
locally.  Usefull for a) editing a topology on one machine, executing it
on another and b) marketing XORP.  The default router model remains quagga.

Bug found by:
Submitted by:
Requested by:
Reviewed by:
Approved by:
Obtained from: