netgdb: Fix netgdb double ack, print proxy address

• Fixes netgdb's double ack
• Moving ack responsibility to debugnet, decision to ack made by netdump/netgdb.
• Finish responsibility moved to debugnet, new finish handler.
• netgdb now prints the address to connect to in case the user doesn't have access to the proxy machine.

Sponsored by: Dell EMC
Reviewed By: markj, bdrewery (earlier version)
Differential Revision: https://reviews.freebsd.org/D40064