This takes another approach at binding FHS directory structure. We
now bind-mount all the root filesystem to directory "/host" in the target tree.
From that we symlink all the directories into the tree if they do not already
exist in FHS structure.
This probably makes `CHROOTENV_EXTRA_BINDS` unnecessary -- its main usecase was
to add bound directories from the host to the sandbox, and we not just symlink
all of them. I plan to get some feedback on its usage and maybe deprecate it.
This also drops old `buildFHSChrootEnv` infrastructure. The main problem with it
is it's very difficult to unmount a recursive-bound directory when mount is not
sandboxed. This problem is a bug even without these changes -- if
you have for example `/home/alice` mounted to somewhere, you wouldn't see
it in `buildFHSChrootEnv` now. With the new directory structure, it's
impossible to use regular bind at all. After some tackling with this I realized
that the fix would be brittle and dangerous (if you don't unmount everything
clearly and proceed to removing the temporary directory, bye-bye fs!). It also
probably doesn't worth it because I haven't heard that someone actually uses it
for a long time, and `buildFHSUserEnv` should cover most cases while being much
more maintainable and safe for the end-user.
Login mode can cause hidden problems, e.g. #12406. Generally we don't want
to read user's .bash_profile when we don't start an interactive shell inside
a chroot.
Previously is was assumed that bash was in the path when calling the
environment setup script. This changes all of the references of bash to
be absolute paths so that the user doesn't have to worry about the
environment they call it with.
