To people upgrading from before r363679

If you are upgrading across r363679, you may have installworld fail, as
documented in UPDATING.

I have a fix (that requires a trip through buildworld) that's under review
at https://reviews.freebsd.org/D25967 . The changes are likely good, but
comments likely need updating.

The short version is that we purposely use old libraries to install the
system. We created a symbolic link to a bunch of binaries on the system and
once installworld installs one of them, we get the error. The workaround
works because we copy libc.so before doing the installworld, so now we're
running with a new libc.so with new binaries, which works. My fix always
copies and never symlinks. The symbolic link stuff is too fragile.

With it, I've done one system, but I'd appreciate reports (on the code
review if possible, to me in email if not) of people who have success
upgrading with this. If you've already run installworld and hit the
undefined symbol, it's too late for you to help me test (since re-running
is the same as hitting to test is the same as the workaround and so it will
work even if my workaround is busted).

Some history: This was introduced about 2 years ago. Prior to that, we
always copied binaries for the install.

Warner