MFC r319219: add a sanity check before installworld on the running system

FreeBSD does not guarantee kernel forward compatibility (that is,
running a newer userland on an older kernel). The documented upgrade
procedure specifies that installkernel should be performed, followed by
a reboot and then installworld. As a sanity check when installing onto
the running system (DESTDIR is / or unset), attempt to run "sh echo OK"
using rescue from the objdir. If rescue fails (e.g., because the system
has not been rebooted and the "old" kernel lacks a system call required
by the to-be-installed world), abort the installation.

This was added to avoid ino64 foot-shooting in HEAD, but is generally
useful for any upgrade case adding new syscalls.

Sponsored by: The FreeBSD Foundation