install: Warn if system is not merged-usr
In Bug#978636, the CTTE decided that booksworm will be merged-usr-only, hence add a warning to APT to inform people about unsupported systems so they get a chance to fix them up.
In Bug#978636, the CTTE decided that booksworm will be merged-usr-only, hence add a warning to APT to inform people about unsupported systems so they get a chance to fix them up.