Kernel/LibC: Implement sched_* functionality to set/get process priority

Right now, we allow anything inside a user to raise or lower any other process's
priority. This feels simple enough to me. Linux disallows raising, but
that's annoying in practice.

Kernel/UnixTypes.h

@@ -400,3 +400,7 @@ struct iovec {
     void* iov_base;
     size_t iov_len;
 };
+
+struct sched_param {
+    int sched_priority;
+};