timeval: Import ctypes Python library within a try statement.

Older versions of Python do not have ctypes as a default installed
package. This patch puts the 'import ctypes' statement inside a try
statement.

This fixes a bug introduced by commit 8396f (timeval: Use monotonic
time in OVS Python timeval library).

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>
ofp-errors: Duplicate instruction error

Add OFPERR_OFPBIC_DUP_INST (type = OFPET_BAD_INSTRUCTION, code = 9)
and use it for OpenFlow1.4+.

For OpenFlow1.1 - 1.3 map this error to ONFBIC_DUP_INSTRUCTION
(experimenter = ONF, type = 2600) which is proposed in
OpenFlow enhancement proposal EXT-260 "Add error
code for duplicate instruction.".

Previously ONFBIC_DUP_INSTRUCTION was used for OpenFlow1.3+.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>