add support for thread scheduling (POSIX TPS option)

linux's sched_* syscalls actually implement the TPS (thread
scheduling) functionality, not the PS (process scheduling)
functionality which the sched_* functions are supposed to have.
omitting support for the PS option (and having the sched_* interfaces
fail with ENOSYS rather than omitting them, since some broken software
assumes they exist) seems to be the only conforming way to do this on
linux.

src/process/posix_spawnattr_sched.c

@@ -0,0 +1,25 @@
+#include <spawn.h>
+#include <sched.h>
+#include <errno.h>
+
+int posix_spawnattr_getschedparam(const posix_spawnattr_t *restrict attr,
+	struct sched_param *restrict schedparam)
+{
+	return ENOSYS;
+}
+
+int posix_spawnattr_setschedparam(posix_spawnattr_t *restrict attr,
+	const struct sched_param *restrict schedparam)
+{
+	return ENOSYS;
+}
+
+int posix_spawnattr_getschedpolicy(const posix_spawnattr_t *restrict attr, int *policy)
+{
+	return ENOSYS;
+}
+
+int posix_spawnattr_setschedpolicy(posix_spawnattr_t *restrict attr, int policy)
+{
+	return ENOSYS;
+}