This is needed because on a mounted `bootc` container `setfiles`
without excluding `/sysroot` will create many warnings like:
```
setfiles: conflicting specifications for /run/osbuild/tree/sysroot/ostree/repo/objects/00/0ef9ada2ee87792e8ba21afd65aa00d79a1253018832652b8694862fb80e84.file and /run/osbuild/tree/usr/lib/firmware/cirrus/cs35l41-dsp1-spk-prot-103c8b8f-r1.bin.xz, using system_u:object_r:lib_t:s0.
```
but simply excluding this dir fixes them.
This is basically a re-implementation of `setfilecon(3)` minus the
translation of human readable context to raw context. Add test for
the new function.
Add a small wrapper around the setfiles(8) utility that can be used
to set the security context fields on one or multiple provided paths,
given a specification. The root of the file system tree can be given
via `root` and all elements of `paths` will be interpreted as
relative to that root.
Add a helper, `parse_config`, to parse a selinux configuration file,
see selinux(8), and return a dictionary containing the configuration
data in key, value pairs. This, in turn, can be fed into the other
helper method, `config_get_policy`, to get the effective policy or
`None` if SELinux is disabled or the policy type is not configured.
Add a new test suite that checks the basic functionality of the
helpers above.
