vlib: Skip core 0 if workers are configured.

vlib: Skip core 0 if workers are configured.

According to the description in the startup.conf, the assignment
of worker threads starts with the lcore following the main_lcore.
A non-zero skip_cores will correctly achieve this assignment.
However, prior to this patch when workers are assigned, the code
picks up and assigns core 0 even thought it shouldn't.
This patch determins if a non-zero number of workers are desired
and if so, marks CPU unavailable for a worker assignment.

Type: fix

Change-Id: I1fdf73a6f218dcbf146fda2efc90c553f7cd6d20
Signed-off-by: Jon Loeliger <jdl@netgate.com>
(cherry picked from commit 4a06846dd668d7f687e6770215c38e8feb5f1740)