Add files/changes missing from previous MFC at 219925

Sponsored by:	The FreeBSD Foundation
Add changes missing from previous MFC at r219925

(Parts of) r217358, r218010, r218152, r218153, and r218155 were missing
from the projects/sv branch.
Change the "netdump" DDB command to call doadump() when netdump is enabled,
rather than calling netdumpsys() directly. Otherwise the context of the
invoking thread is not saved in dumppcb and kgdb is unable to extract a
backtrace for it.

As a side effect, this change ensures that the kernel will attempt to dump
core using netdump if a user runs the panic command from within DDB.
Previously, this would simply result in a reboot if a dump device had not
been configured (which is generally the case when netdump is enabled).

Reviewed by:	rstone
Sponsored by:	Sandvine Inc.