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>

libcontainer/container_linux.go

@@ -709,6 +709,7 @@ func (c *Container) newInitConfig(process *Process) *initConfig {
 		Rlimits:          c.config.Rlimits,
 		IOPriority:       c.config.IOPriority,
 		Scheduler:        c.config.Scheduler,
+		CPUAffinity:      c.config.ExecCPUAffinity,
 		CreateConsole:    process.ConsoleSocket != nil,
 		ConsoleWidth:     process.ConsoleWidth,
 		ConsoleHeight:    process.ConsoleHeight,
@@ -737,6 +738,9 @@ func (c *Container) newInitConfig(process *Process) *initConfig {
 	if process.Scheduler != nil {
 		cfg.Scheduler = process.Scheduler
 	}
+	if process.CPUAffinity != nil {
+		cfg.CPUAffinity = process.CPUAffinity
+	}
 
 	// Set misc properties.