Change dumpon(8)'s handling of -g.

Rather than using a special value to denote "use the default router",
treat the absence of the -g option to mean the same thing. The
in-kernel netdump client will always attempt to reach the server
directly before falling back to the configured gateway anyway. This
change makes it cleaner to support a hostname value for -g.

Reviewed by: cem
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D18025