Prefetched sets are stored by root, but are read by an unprivileged

Prefetched sets are stored by root, but are read by an unprivileged
user during the actual install phase. Add a check to the selection
of the prefetch area to ensure the unprivleged user can read files
from there.

Move the selection logic to its own function prefetcharea_fs_list()
which returns a unique list of candidate filesystems, or with rc=1.
While at it, remove /var/tmp from the potential list of candidates
because it's a symbolic link to /tmp since 2014.

Change install_files() to loop over the candidates and check if it
can read a test file, otherwise clean up and skip this filesystem.

Problem found by benno@ whose upgrade failed due to /usr/obj being
owned by build:wobj and permissions that prevented the unprivileged
user to read the prefetched set files from there.

discussed with and OK benno@ halex@ tb@