[kernel] Use map for process handles

Replace linearly-indexed vector of handles with new kutil::map. Also
provide thread::current() and process::current() accessors so that every
syscall doesn't need to include the scheduler to deduce the current
process.

src/kernel/objects/thread.cpp

@@ -1,4 +1,5 @@
 #include "j6/signals.h"
+#include "cpu.h"
 #include "log.h"
 #include "objects/thread.h"
 #include "objects/process.h"
@@ -40,6 +41,12 @@ thread::from_tcb(TCB *tcb)
 	return reinterpret_cast<thread*>(kutil::offset_pointer(tcb, offset));
 }
 
+thread &
+thread::current()
+{
+	return *bsp_cpu_data.t;
+}
+
 void
 thread::wait_on_signals(kobject *obj, j6_signal_t signals)
 {