runc exec: implement CPU affinity

As per
- https://github.com/opencontainers/runtime-spec/pull/1253
- https://github.com/opencontainers/runtime-spec/pull/1261

CPU affinity can be set in two ways:
1. When creating/starting a container, in config.json's
   Process.ExecCPUAffinity, which is when applied to all execs.
2. When running an exec, in process.json's CPUAffinity, which
   applied to a given exec and overrides the value from (1).

Add some basic tests.

Note that older kernels (RHEL8, Ubuntu 20.04) change CPU affinity of a
process to that of a container's cgroup, as soon as it is moved to that
cgroup, while newer kernels (Ubuntu 24.04, Fedora 41) don't do that.

Because of the above,
 - it's impossible to really test initial CPU affinity without adding
   debug logging to libcontainer/nsenter;
 - for older kernels, there can be a brief moment when exec's affinity
   is different than either initial or final affinity being set;
 - exec's final CPU affinity, if not specified, can be different
   depending on the kernel, therefore we don't test it.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

CHANGELOG.md

@@ -23,6 +23,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
    methods no longer ignore `Process.IOPriority` and `Process.Scheduler`
    settings. (#4585)
 
+### Added
+ * CPU affinity support for `runc exec`. (#4327)
+
 ## [1.2.5] - 2025-02-13
 
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