ofp-actions: Centralize all OpenFlow action code for maintainability.

Until now, knowledge about OpenFlow has been somewhat scattered around the
tree.  Some of it is in ofp-actions, some of it is in ofp-util, some in
separate files for individual actions, and most of the wire format
declarations are in include/openflow.  This commit centralizes all of that
in ofp-actions.

Encoding and decoding OpenFlow actions was previously broken up by OpenFlow
version.  This was OK with only OpenFlow 1.0 and 1.1, but each additional
version added a new wrapper around the existing ones, which started to
become hard to understand.  This commit merges all of the processing for
the different versions, to the extent that they are similar, making the
version differences clearer.

Previously, ofp-actions contained OpenFlow encoding and decoding, plus
ofpact formatting, but OpenFlow parsing was separated into ofp-parse, which
seems an odd division.  This commit moves the parsing code into ofp-actions
with the rest of the code.

Before this commit, the four main bits of code associated with a particular
ofpact--OpenFlow encoding and decoding, ofpact formatting and parsing--were
all found far away from each other.  This often made it hard to see what
was going on for a particular ofpact, since you had to search around to
many different pieces of code.  This commit reorganizes so that all of the
code for a given ofpact is in a single place.

As a code refactoring, this commit has little visible behavioral change.
The update to ofproto-dpif.at illustrates one minor bug fix as a side
effect: a flow that was added with the action "dec_ttl" (a standard
OpenFlow action) was previously formatted as "dec_ttl(0)" (using a Nicira
extension to specifically direct packets bounced to the controller because
of too-low TTL), but after this commit it is correctly formatted as
"dec_ttl".

The other visible effect is to drop support for the Nicira extension
dec_ttl action in OpenFlow 1.1 and later in favor of the equivalent
standard action.  It seems unlikely that anyone was really using the
Nicira extension in OF1.1 or later.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>