Replace home-grown CTL IO allocator with UMA.

Old allocator created significant lock congestion protecting its lists
of preallocated I/Os, while UMA provides much better SMP scalability.
The downside of UMA is lack of reliable preallocation, that could guarantee
successful allocation in non-sleepable environments.  But careful code
review shown, that only CAM target frontend really has that requirement.
Fix that making that frontend preallocate and statically bind CTL I/O for
every ATIO/INOT it preallocates any way.  That allows to avoid allocations
in hot I/O path.  Other frontends either may sleep in allocation context
or can properly handle allocation errors.

On 40-core server with 6 ZVOL-backed LUNs and 7 iSCSI client connections
this change increases peak performance from ~700K to >1M IOPS!  Yay! :)

MFC after:	1 month
Sponsored by:	iXsystems, Inc.
Add ip_gre.h to ObsoleteFiles.inc.

Sponsored by:	Yandex LLC
Make it possible to specify flags for autofs daemons in rc.conf.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Add a sysctl `net.link.tap.deladdrs_on_close' to configure whether tap
should delete configured addresses and routes when the interface is
closed.  Default is enabled (preserve current behaviour).

MFC after:	1 week
Add busdma sync ops before reading and after modifying the descriptor rings.

This was previously working by accident because BUSDMA_COHERENT_MEMORY has
always been set to strongly-ordered on arm.  Now we're moving towards
normal-uncacheable (what might be called write-combining on other platforms)
and using the proper sync ops will be more important.  Of course, that
opens the question of just what is the "proper" sync op for shared
concurrent dma access as opposed to accesses where the handoff of control
of the memory has well-defined sequence points that match the available
busdma sync operations.