[kernel] Create process kernel object

Re-implent the concept of processes as separate from threads, and as a
kobject API object. Also improve scheduler::prune which was doing some
unnecessary iterations.

src/include/j6/signals.h

@@ -6,6 +6,7 @@
 #define j6_signal_no_handles		(1ull << 0)
 
 // Signals 16-47 are defined per-object-type
+#define j6_signal_process_exit		(1ull << 16)
 
 // Signals 48-63 are user-defined signals
 #define j6_signal_user0				(1ull << 48)