[INET] Create enum of ECN bits

Hi Dave:

This patch is a preparation for an update of the ECN encap/decap
code with respect to RFC3168.

It creates an enum of the four code-points defined by RFC3168
and uses them throughout the inet_ecn.h file.

The only non-trivial bit is in IP_ECN_set_ce/IP6_ECN_set_ce where
the patch uses INET_ECN_CE instead of 1.  This is OK as those
functions assume that the ECT bit is already set.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
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