[kernel] Add userspace threading

Implement the syscalls necessary for threads to create other threads in
their same process. This involved rearranging a number of syscalls, as
well as implementing object_wait and a basic implementation of a
process' list of handles.

src/include/j6/signals.h

@@ -6,8 +6,13 @@
 #define j6_signal_no_handles		(1ull << 0)
 
 // Signals 16-47 are defined per-object-type
+
+// Process signals
 #define j6_signal_process_exit		(1ull << 16)
 
+// Thread signals
+#define j6_signal_thread_exit		(1ull << 16)
+
 // Signals 48-63 are user-defined signals
 #define j6_signal_user0				(1ull << 48)
 #define j6_signal_user1				(1ull << 49)