Overhaul if_gre(4).

Split it into two modules: if_gre(4) for GRE encapsulation and
if_me(4) for minimal encapsulation within IP.

gre(4) changes:
* convert to if_transmit;
* rework locking: protect access to softc with rmlock,
  protect from concurrent ioctls with sx lock;
* correct interface accounting for outgoing datagramms (count only payload size);
* implement generic support for using IPv6 as delivery header;
* make implementation conform to the RFC 2784 and partially to RFC 2890;
* add support for GRE checksums - calculate for outgoing datagramms and check
  for inconming datagramms;
* add support for sending sequence number in GRE header;
* remove support of cached routes. This fixes problem, when gre(4) doesn't
  work at system startup. But this also removes support for having tunnels with
  the same addresses for inner and outer header.
* deprecate support for various GREXXX ioctls, that doesn't used in FreeBSD.
  Use our standard ioctls for tunnels.

me(4):
* implementation conform to RFC 2004;
* use if_transmit;
* use the same locking model as gre(4);

PR:		164475
Differential Revision:	D1023
No objections from:	net@
Relnotes:	yes
Sponsored by:	Yandex LLC
Remove unused assignments, noticed by Clang analyzer.

MFC after:	1 week
Avoid panic in ofwbus caused by not released resource list entry

After resource allocation and release, resource list entry
stays non-NULL. This causes panic in ofwbus_alloc_resource()
on subsequent resource allocation.
Clean appropriate list entry on release to avoid this.

Obtained from:	Semihalf
Reviewed by:	ian
Sponsored by:	The FreeBSD Foundation
Simplify assembler in ivy.c.  Move the copying of the random bits into
buffer from asm to C, which reduces amount of arguments for inline asm
and simplifies constraints.  Use unsigned types consistently.

Submitted by:	bde
Approved by:	secteam (delphij)
Reviewed by:	markm
MFC after:	1 week
Add DEV_MODULE_ORDERED().

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Fix random.ko module.
- Remove duplicated sources between standard part of the kernel and
  module.  In particular, it caused duplicated lock initialization and
  sysctl registration, both having bad consequences.
- Add missed source files to module.
- Static part of the kernel provides randomdev module, not
  random_adaptors.  Correct dependencies.
- Use cdev modules declaration macros.

Approved by:	secteam (delphij)
Reviewed by:	markm
Fix LUN resize broken by r272911 commit.

MFC after:	3 days
sh(1): Mention portability issue with shifting zero positional parameters.

Per Austin Group issue #459, shifting zero positional parameters may or may
not be considered an operand error (which causes the shell to exit in most
cases).