implement dummy pthread_attr_[gs]etschedparam functions

for some reason these functions are not shaded by the PS/TPS option in
POSIX, so presumably they are mandatory, even though the functionality
they offer is optional. for now, provide them in case any programs
depend on their existence, but disallow any priority except the
default.

src/thread/pthread_attr_getschedparam.c

+#include "pthread_impl.h"
+
+int pthread_attr_getschedparam(const pthread_attr_t *a, struct sched_param *param)
+{
+	param->sched_priority = 0;
+	return 0;
+}

src/thread/pthread_attr_setschedparam.c

+#include "pthread_impl.h"
+
+int pthread_attr_setschedparam(pthread_attr_t *a, const struct sched_param *param)
+{
+	if (param->sched_priority) return ENOTSUP;
+	return 0;
+}