Improve panic error messages.

The previous sys_panic() output tended to scroll important stuff
off the top of the screen.

k-hardware.cc

@@ -322,7 +322,7 @@ static void vpanic(uintptr_t rsp, uintptr_t rbp, uintptr_t rip,
                    const char* format, va_list val) {
     panicking = true;
 
-    cursorpos = CPOS(24, 80);
+    cursorpos = CPOS(24, 0);
     if (format) {
         // Print panic message to both the screen and the log
         error_printf(-1, COLOR_ERROR, "PANIC: ");

kernel.cc

@@ -180,7 +180,7 @@ uintptr_t proc::syscall(regstate* regs) {
         return 0;
 
     case SYSCALL_PANIC:
-        panic(nullptr);
+        panic_at(0, 0, 0, "process %d called sys_panic()", id_);
         break;                  // will not be reached
 
     case SYSCALL_GETPID:

p-testhalt.cc

@@ -20,6 +20,7 @@ void process_main() {
     } else {
         sys_msleep(300);
         console_printf("testhalt succeeds if you see 0-2 above and QEMU exits after a second\n");
+        console_printf("(QEMU will only exit if you ran with `HALT=1`)\n");
         sys_msleep(1000);
         sys_exit(0);
     }