rescue: Add fetch(1) to the rescue tool.
After a failed upgrade, having fetch(1) on a system that is
physically unnreachable would be very useful to download files
required to get the OS back up and functional.
On my system this adds 589,824 bytes (3.8%) to the binary size.
PR: 266224
Reported by: Dan Mahoney
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38193