Add new a M_START() mbuf macro that returns a pointer to the start of
an mbuf's storage (internal or external).

Add a new M_SIZE() mbuf macro that returns the size of an mbuf's
storage (internal or external).

These contrast with m_data and m_len, which are with respect to data
in the buffer, rather than the buffer itself.

Rewrite M_LEADINGSPACE() and M_TRAILINGSPACE() in terms of M_START()
and M_SIZE().

This is done as we currently have many instances of using mbuf flags
to generate pointers or lengths for internal storage in header and
regular mbufs, as well as to external storage. Rather than replicate
this logic throughout the network stack, centralising the
implementation will make it easier for us to refine mbuf storage.
This should also help reduce bugs by limiting the amount of
mbuf-type-specific pointer arithmetic.  Followup changes will
propagate use of the macros throughout the stack.

M_SIZE() conflicts with one macro in the Chelsio driver; rename that
macro in a slightly unsatisfying way to eliminate the collision.

MFC after:	3 days
Obtained from:	jeff (with enhancements)
Sponsored by:	EMC / Isilon Storage Division
Reviewed by:	bz, glebius, np
Differential Revision:	https://reviews.freebsd.org/D753
* constify argument of in6_addrscope();
* use IN6_IS_ADDR_XXX() macro instead of hardcoded values;
* for multicast addresses just return scope value, the only exception
  is addresses with 0x0F scope value (RFC 4291 p2.7.0);

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Rename pmap_kenter_temp to pmap_kenter_temporary to be consistent with the
other architectures with this function.

Submitted by:	Svatopluk Kraus <onwahe at gmail.com>
Submitted by:	Michal Meloun <meloun at miracle.cz>
- Add $netif_ipexpand_max to specify the upper limit for the number of
  addresses generated by an address range specification.  The default
  value is 2048.  This can be increased by setting $netif_ipexpand_max
  in rc.conf.

- Fix warning messages when an address range spec exceeds the upper limit.

PR:	186841
Introduce new scope related functions.

* new macro to remove magic number - IPV6_ADDR_SCOPES_COUNT;
* sa6_checkzone() - this function checks sockaddr_in6 structure
  for correctness of sin6_scope_id. It also can fill correct
  value sometimes.
* in6_getscopezone() - this function returns scope zone id for
  specified interface and scope.
* in6_getlinkifnet() - this function returns struct ifnet for
  corresponding zone id of link-local scope.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Minimize usage of leaked definitions from pte.h
Rename pmap_kenter_temp() to commonly used pmap_kenter_temporary().
Note: minidump still needs next cleanup.
Cleanup switch.S. Simplify frame structures used in system.
Simplify TLS handling.
Since TLS pointer cannot be changed directly from userland is not necessary to save it on each task switch.
Add definition for CP15 registers. Define new inline TLB and cache functions.
Cleanup exception.S. Prepare for later unification of data_abort_handler() and prefetch_abort_handler().
First step in cleanup of locore.S. Fix initial stack allocation. Rename initial page table and use it for startup of secondary cores too. Same page table can be used for CPU hot plug and/or power management later.
Add new ARMv6 specific trap-v6.c.
Add new pmap code.