ofp-actions: Support OF1.5 (draft) masked Set-Field, merge with reg_load.

ofp-actions: Support OF1.5 (draft) masked Set-Field, merge with reg_load.

OpenFlow 1.5 (draft) extends the OFPAT_SET_FIELD action originally
introduced in OpenFlow 1.2 so that it can set not just entire fields but
any subset of bits within a field as well.  This commit adds support for
that feature when OpenFlow 1.5 is used.

With this feature, OFPAT_SET_FIELD becomes a superset of NXAST_REG_LOAD.
Thus, this commit merges the implementations of the two actions into a
single ofpact_set_field.

ONF-JIRA: EXT-314
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>