Change local variable tso_segsz to tsosegsz to avoid mbuf.h macro conflicts.

Sponsored by:	The FreeBSD Foundation
Restructure the mbuf pkthdr to make it fit for upcoming capabilities and
features.  The changes in particular are:

o Remove rarely used "header" pointer and replace it with a 64bit protocol/
  layer specific union PH_loc for local use.  Protocols can flexibly overlay
  their own 8 to 64 bit fields to store information while the packet is
  worked on.

o Mechanically convert IP reassembly, IGMP/MLD and ATM to use pkthdr.PH_loc
  instead of pkthdr.header.

o Extend csum_flags to 64bits to allow for additional future offload
  information to be carried (e.g. iSCSI, IPsec offload, and others).

o Move the RSS hash type enumerator from abusing m_flags to its own 8bit
  rsstype field.  Adjust accessor macros.

o Add cosqos field to store Class of Service / Quality of Service information
  with the packet.  It is not yet supported in any drivers but allows us to
  get on par with Cisco/Juniper in routing applications (plus MPLS QoS) with
  a modernized ALTQ.

o Add four 8 bit fields l[2-5]hlen to store the relative header offsets
  from the start of the packet.  This is important for various offload
  capabilities and to relieve the drivers from having to parse the packet
  and protocol headers to find out location of checksums and other
  information.  Header parsing in drivers is a lot of copy-paste and
  unhandled corner cases which we want to avoid.

o Add another flexible 64bit union to map various additional persistent
  packet information, like ether_vtag, tso_segsz and csum fields.
  Depending on the csum_flags settings some fields may have different usage
  making it very flexible and adaptable to future capabilities.

o Restructure the CSUM flags to better signify their outbound (down the
  stack) and inbound (up the stack) use.  The CSUM flags used to be a bit
  chaotic and rather poorly documented leading to incorrect use in many
  places.  Bring clarity into their use through better naming.
  Compatibility mappings are provided to preserve the API.  The drivers
  can be corrected one by one and MFC'd without issue.

o The size of pkthdr stays the same at 48/56bytes (32/64bit architectures).

Sponsored by:	The FreeBSD Foundation
Add mtodo(m, o) macro taking an additional offset into the mbuf data section.

Sponsored by:	The FreeBSD Foundation
sh: Reject ++ and -- in arithmetic.

POSIX does not require ++ and -- in arithmetic. It is probably more useful
to reject them than to treat ++x and --x as x silently.

Note that the behaviour of increment and decrement can be obtained via
(x+=1), ((x+=1)-1), (x-=1) and ((x-=1)+1).

PR:		bin/176444