lints: Add --fix, support with tmpfiles

[cgwalters]

This needs some docs and some container tests but yeah

$ cat Dockerfile
FROM localhost/bootc
RUN <<EORUN
set -xeuo pipefail
mkdir -p /var/lib/some-test-dir
bootc container lint --fix
EORUN
$ podman build ...
+ bootc container lint --fix
tmpfiles.d: Generated: usr/lib/tmpfiles.d/bootc-autogenerated-var-0.conf with entries: 1
Checks passed: 11
Checks skipped: 1

But the big thing I realized here that makes things WAY messier is that actually we need to support retaining some of the /var subdirs...like /var/tmp and /var/lib/rpm-state as one wants them in the container too. Tentatively thinking something like

cat /usr/lib/bootc/tmpfiles.d/foo.conf
# Temporary directories we ensure are always present in the container too
/var/tmp

So what'd be cool about this is that at the end of a build one could switch from

rm -rf /var/tmp/* /var/lib/{dnf,rhsm} /var/cache/*

to

rm /var/*
bootc container lint --fix

And that'd automatically regenerate everything in /var needed.

But, mmm, it may drive us really to have something like bootc container commit that is defined to do both of those things?