Merge from projects/sendfile:

o Provide a new VOP_GETPAGES_ASYNC(), which works like VOP_GETPAGES(), but
  doesn't sleep. It returns immediately, and will execute the I/O done handler
  function that must be supplied as argument.
o Provide VOP_GETPAGES_ASYNC() for the FFS, which uses vnode_pager.
o Extend pagertab to support pgo_getpages_async method, and implement this
  method for vnode_pager.

Reviewed by:	kib
Tested by:	pho
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
HBSD: fix bhyveload memory allocation error

Details:

We changed the VM_MIN_ADDRESS from 0 to 64k to prevent mapping bellow the
64k address from user-space, this change prevents NULL pointer derefs.
In vm_* functions checking for MAX(sv_minuser, PAGE_SIZE) in our case
this always evaluated to 64k, but this check requiered only for
user-space programs.
In bhyve case, in the vmm module created new vmspace, but this must to
be beginning from address 0 to properly emulate things. In the original
implentation they used VM_MIN_ADDRESS to set this addres, but we
(HardenedBSD) changed this constant, to resolve this, change the
VMSPACE_ALLOC() parameter from VM_MIN_ADDRESS to 0.

Tested on Core i5-4200U CPU with FreeBSD installer.

Signed-off-by: Oliver Pinter <oliver.pinter@hardenedbsd.org>
Don't use the synchronization endpoint unless referenced by the
isochronous endpoint descriptor used for the data transfers, hence the
synchronization feature might not be supposed to be supported [yet].
This makes seamless playback synced with the USB HOST clock work with
the DN32-USB module for Midas audio systems and possibly other similar
products from Klark Teknik.

MFC after:	1 week
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/segvguard
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/log
SIGNAL ORDERING: priority ordering changes.
github-issue: #56
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/userlandenhanced
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/pie
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/segvguard' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/log' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/userlandenhanced' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/pie' into hardened/current/unstable