Certain tools, e.g. compilers, are customarily prefixed with the name of
their target platform so that multiple builds can be used at once
without clobbering each other on the PATH. I was using identifiers named
`prefix` for this purpose, but that conflicts with the standard use of
`prefix` to mean the directory where something is installed. To avoid
conflict and confusion, I renamed those to `targetPrefix`.
Would be just mdadm: 3.3.4 -> 4.0, but it doesn't look like there are
urgent bugfixes, and it is a major release, and wrong RAID handling
kills data, so let's let the early adopters test it a bit.
