[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)

src/include/j6/types.h

@@ -18,3 +18,5 @@ typedef uint64_t j6_signal_t;
 
 /// The rights of a handle/capability are a bitmap of 64 possible rights
 typedef uint64_t j6_rights_t;
+
+#define j6_handle_invalid 0xffffffff

src/include/kernel_memory.h

@@ -16,9 +16,6 @@ namespace memory {
 	/// Offset from physical where page tables are mapped.
 	constexpr uintptr_t page_offset = 0xffffc00000000000;
 
-	/// Initial process thread's stack address
-	constexpr uintptr_t initial_stack = 0x0000800000000000;
-
 	/// Initial process thread's stack size, in pages
 	constexpr unsigned initial_stack_pages = 1;