Document XENHVM and xenpci are mutually inclusive.

Submitted by:   gibbs
Approved by:    re (delphij)
Sponsored by:   The FreeBSD Foundation
Fix a bug that would occur in application death when debugging was enabled
if you ended up in f_dialog_*_constrain() (indirectly, of course) with a
purposefully-set NULL height, width, or rows parameter (because you didn't
care to have that attribute calculated). Fix typo in a comment while here.

Approved by:	re (glebius)
Add $VAR_ZFSINTERACTIVE (zfsInteractive) and new f_zfsinteractive() for
determining when a script wants to be nonInteractive but selectively
wants ZFS operations to be *interactive* (this is analgous to already
existing $VAR_NETINTERACTIVE (netInteractive) and f_netinteractive()
used for the same purpose (script wants to be nonInteractive but wants
network operations to be *interactive*).

Approved by:	re (glebius)
Remove extraneous \n.

Noticed by:     Nikolai Lifanov <lifanov@mail.lifanov.com>

Approved by:	re (gjb)
Store descriptions for DEVICE_TYPE_DISK and fix static-defaults mapping.
Add a default description for bhyve block disk device while we're here.

Approved by:	re (glebius)
Remove extraneous (but harmless) leading /.

Noticed by:     Herbert J. Skuhra <hskuhra@eumx.net>
Approved by:	re (gjb)
Add new f_expand_number() (shadowing expand_number(3)).

Approved by:	re (glebius)