Vendor import of OpenPAM Ourouparia.
Add a brief description of CTLFLAG_VNET flag. Since the VIMAGE is
absolutely not documented, I see no reason in long descriptions
here.
Add comments describing r271604 change.

MFC after:	3 days
r271256 fixed one segfault condition but introduced another due to the
wrong operator being used in the tty check.

Reported by:	avg@
MFH:		3 days
Provide the unique implementation for the VOP_GETPAGES() method used
by ffs and ext2fs.  Remove duplicated call to vm_page_zero_invalid(),
done by VOP and by vm_pager_getpages().  Use vm_pager_free_nonreq().

Reviewed by:	alc (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	6 weeks (after r271596)
Upgrade to OpenPAM Ourouparia.
Use generic SYSCTL_* macro instead of deprecated SYSCTL_VNET_*.

Suggested by:	glebius
xen: don't set suspend/resume methods for the PIRQ PIC

The suspend/resume of event channels is already handled by the xen_intr_pic.
If those methods are set on the PIRQ PIC they are just called twice, which
breaks proper resume. This fix restores migration of FreeBSD guests to a
working state.

Sponsored by: Citrix Systems R&D
The private peripheral interrupts start at offset 16, not 0.  Also, use
names rather than inline mystery constants for these offsets.

Pointed out by:	andrew
Remove redundant declaration. vnode.h should be included before vnode_pager.h.