examples/l3fwd: set CLI back to Unix style

examples/l3fwd: set CLI back to Unix style

The l3fwd program became chatty, and the code looks cluttered in
recent versions. DPDK programs should try for Unix (not VMS) style
and not print messages for the options user selected. Also, errors should
be printed on stderr. To make it easier to find code matching error
messages; error strings should be placed in situ rather than saved
as string vaiables.

Fixes: 268888b5b020 ("examples/l3fwd: modularize")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>