Add support for write-actions

Add support for write-actions

Implementation note:

All actions which modify a field are added to the action set
at the point where "set" actions should be added. In general
modifying a field many times is the same as only modifying it
the last time so the implementation simply adds all set actions to
the action set in the order they are specified. However, this breaks
down if two actions modify different portions of the same field.

Some examples.

1. load acting a subfield
2. mod_vlan_vid, mod_vlan_pcp

If this is considered to be a problem one possible solution would be to
either disallow all set actions other than set_field in write_actions.
Another possible solution is prohibit problematic the actions listed above
in write actions.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com simplified and edited the code]
Signed-off-by: Ben Pfaff <blp@nicira.com>