ovs-vswitchd: Better diagnose errors in DPDK command-line options.

With DPDK compiled in, when the --dpdk option was given other than as the
first command-line argument, ovs-vswitchd silently ignored it.  Without
DPDK compiled in, when the --dpdk option was given anywhere, ovs-vswitchd
silently ignored it.  However, in each case any options following --dpdk
were not ignored, and since --dpdk is normally followed by additional
DPDK-specific options, this caused even more confusing trouble.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>
Tested-by: Daniele Di Proietto <ddiproietto@vmware.com>