Allocate four bits from the mbuf flags field to represent the hash

Allocate four bits from the mbuf flags field to represent the hash
type of a software- or hardware-generated hash held in the
mbuf.m_pkthdr.flowid field, and provide accessor macros to easily
clear, set, receive, and test for hash values.  Some of these
constants correspond to RSS hash types, but we don't want to limit
ourselves to that, as a number of other hashing techniques are in
use on hardware supported by FreeBSD.

Mark the M_FLOWID flag as deprecated; I hope to remove this before
9.0, changing drivers and the stack over to using the new
M_HASHTYPEBITS, most likely to use M_HASHTYPE_OPAQUE as we don't yet
want to nail down the KPI for RSS key/bucket management for device
drivers.

MFC after:      3 days
Reviewed by:    bz
Sponsored by:   Juniper Networks, Inc.