Deprecate builds with split-usr, prepare for removal

There is no technical reason to support systems with split-usr, except for
backwards compatibility. Even though systemd itself makes an effort to support
this, many other tools aren't as careful. Despite those efforts, we
(collectively) get it wrong often, because doing it "wrong" on systems with
merged-usr has no consequences. Since almost all developers are on such
systems, any issues are only discovered late. Supporting this split-usr mode
makes both code and documentation more complicated. The split is purely
artificial and has no justification except to allow old installation to not
update. Mechanisms to update existing systems are available though: Fedora
did that in https://fedoraproject.org/wiki/Features/UsrMove, Debian has
the usrmerge package.

The next version of Debian will only support systems with split-usr=false,
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978636#178:

  The Technical Committee resolves that Debian 'bookworm' should
  support only the merged-usr root filesystem layout, dropping support
  for the non-merged-usr layout.

Let's start warning if split-usr mode is used, in preparation to removing the
split in one of the future releases.