bin/install-qa-check.d: add new 90bad-bin-group-write QA check.

System executables that are writable by a non-root user pose a
security risk. Anyone who can write to an executable can change its
behavior. If that executable is later run with elevated privileges
(say, by root, when the machine starts), then the non-root user can
escalate his own privileges to those of the person running the
modified executable.

The 90bad-bin-owner check already addresses one cause for a non-root
user to be able to modify an executable: because he owns it. This
commit adds another check, to ensure that no non-root *groups* have
write access to any system executables. On a "normal" system, all
system executables should be writable only by the super-user's group,
if any. To avoid false-positives, non-"normal" systems (like prefix)
are skipped.

Closes: https://bugs.gentoo.org/629398
1 file changed