nspawn: add support for executing OCI runtime bundles with nspawn

nspawn: add support for executing OCI runtime bundles with nspawn

This is a pretty large patch, and adds support for OCI runtime bundles
to nspawn. A new switch --oci-bundle= is added that takes a path to an
OCI bundle. The JSON file included therein is read similar to a .nspawn
settings files, however with a different feature set.

Implementation-wise this mostly extends the pre-existing Settings object
to carry additional properties for OCI. However, OCI supports some
concepts .nspawn files did not support yet, which this patch also adds:

1. Support for "masking" files and directories. This functionatly is now
   also available via the new --inaccesible= cmdline command, and
   Inaccessible= in .nspawn files.

2. Support for mounting arbitrary file systems. (not exposed through
   nspawn cmdline nor .nspawn files, because probably not a good idea)

3. Ability to configure the console settings for a container. This
   functionality is now also available on the nspawn cmdline in the new
   --console= switch (not added to .nspawn for now, as it is something
   specific to the invocation really, not a property of the container)

4. Console width/height configuration. Not exposed through
   .nspawn/cmdline, but this may be controlled through $COLUMNS and
   $LINES like in most other UNIX tools.

5. UID/GID configuration by raw numbers. (not exposed in .nspawn and on
   the cmdline, since containers likely have different user tables, and
   the existing --user= switch appears to be the better option)

6. OCI hook commands (no exposed in .nspawn/cmdline, as very specific to
   OCI)

7. Creation of additional devices nodes in /dev. Most likely not a good
   idea, hence not exposed in .nspawn/cmdline. There's already --bind=
   to achieve the same, which is the better alternative.

8. Explicit syscall filters. This is not a good idea, due to the skewed
   arch support, hence not exposed through .nspawn/cmdline.

9. Configuration of some sysctls on a whitelist. Questionnable, not
   supported in .nspawn/cmdline for now.

10. Configuration of all 5 types of capabilities. Not a useful concept,
    since the kernel will reduce the caps on execve() anyway. Not
    exposed through .nspawn/cmdline as this is not very useful hence.

Note that this only implements the OCI runtime logic itself. It does not
provide a runc-compatible command line tool. This is left for a later
PR. Only with that in place tools such as "buildah" can use the OCI
support in nspawn as drop-in replacement.

Currently still missing is OCI hook support, but it's already parsed and
everything, and should be easy to add. Other than that it's OCI is
implemented pretty comprehensively.

There's a list of incompatibilities in the nspawn-oci.c file. In a later
PR I'd like to convert this into proper markdown and add it to the
documentation directory.