datapath: Add support for CAPWAP UDP transport.

datapath: Add support for CAPWAP UDP transport.

Add support for the transport portion of the CAPWAP protocol as
an alternative to GRE for L2 over L3 tunneling.  This is not
full support for the CAPWAP protocol.  CAPWAP covers management
of wireless access points and describes a control protocol for
setting those devices up.  It also describes a data plane protocol
that allows packets to be tunneled to a controller for inspection.
This data plane protocol is the only component covered by this
commit.

Signed-off-by: Jesse Gross <jesse@nicira.com>