Remove -r toggle and generally be less smart.

Remove -r toggle and generally be less smart.

The default is to install the next release. Snapshots are only
installed when invoked with -s.

The logic on what to do per default got out of hand and it was very
difficult to reason about what sysupgrade(8) actually did. deraadt@
then suggested that we should dumb it all down, sysupgrade(8) is there
to upgrade from one release to the next. More advance usage needs to
be requested by the user.

With all this simplification we can now be a bit more smart to work
out what the next release is. With that, snapshots right before a
release can be sysupgrade(8)'ed to the official release.

OK sthen on a previous version that was much more complicated but
allowed shortly-before-release -> release upgrade

testing sthen on this version

Guidance, prodding & OK deraadt