[NETEM]: use PSCHED_LESS

Convert netem to use PSCHED_LESS and warn if requeue fails.
With some of the psched clock sources, the subtraction doesn't
work always work right without wrapping.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>