process-util: Add support SCHED_EXT scheduling policy

Allow CPUSchedulingPolicy to be set to "ext". SCHED_EXT is a new
scheduling policy in Linux v6.12 that allows processes to be scheduled
using custom BPF schedulers instead of the default in-kernel ones.

Selectively setting the SCHED_EXT policy is useful for systems running
in "partial mode" where not all processes are run using a custom
scheduler.

Fallback to SCHED_OTHER and print an error message for systems where
SCHED_EXT isn't available.

src/include/override/sched.h

@@ -55,3 +55,11 @@ int __clone2(int (*fn)(void *), void *stack_base, size_t stack_size, int flags,
 int missing_sched_setattr(pid_t pid, struct sched_attr *attr, unsigned flags);
 #  define sched_setattr missing_sched_setattr
 #endif
+
+/* f0e1a0643a59bf1f922fa209cec86a170b784f3f (6.12),
+ * defined in sched.h in glibc since glibc-2.41. */
+#ifndef SCHED_EXT
+#  define SCHED_EXT 7
+#else
+static_assert(SCHED_EXT == 7, "");
+#endif