sysupgrade in src?

[nia <nia%NetBSD.org@localhost>]

There's been a few discussions over the years about the need for a
usable-over-ssh binary upgrade tool for NetBSD, but they haven't gone
very far.  Users ask fairly frequently, and other than sysupgrade, the
best method is to implement sysupgrade manually with ftp and tar (and
this is required for binary upgrades on some NetBSD ports that lack
installer images, so I've had to add it to the official installation
and upgrade notes).

I'd like to import sysupgrade into src, since we're basically
maintaining it in pkgsrc due to jmmv no longer accepting pull
requests (and requiring Google CLA to be signed!), and juggling
pkgsrc patches so it keeps pace with NetBSD changes like xz sets
isn't most ideal.

It doesn't have any dependencies that aren't already in
NetBSD, other than shtk, which is small enough as a shell
scripting library that I'd simply bundle it with sysupgrade.
Other than that it relies on sh, ftp, tar, etcupdate,
postinstall, and ATF.