Add ddb(4) 'netdump' command to netdump a core without preconfiguration

Add ddb(4) 'netdump' command to netdump a core without preconfiguration

Add a 'X -s <server> -c <client> [-g <gateway>] -i <interface>' subroutine
to the generic debugnet code.  The imagined use is both netdump, shown here,
and NetGDB (vaporware).  It uses the ddb(4) lexer, with some new extensions,
to parse out IPv4 addresses.

'Netdump' uses the generic debugnet routine to load a configuration and
start a dump, without any netdump configuration prior to panic.

Loosely derived from work by:	John Reimer <john.reimer AT emc.com>
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D21460