Without --allow-root spack cannot push binaries that contain paths in
binaries. This flag is almost always needed, so there is no point of
requiring users to spell it out.
Even without --allow-root, rpaths would still have to be patched, so the
flag is not there to guarantee binaries are not modified on install.
This commit makes --allow-root the default, and drops the code
required for it. It also deprecates `spack buildcache preview`, since
the command made sense only with --allow-root.
As a side effect, Spack no longer depends on binutils for relocation
5b23c5dcc0