syscalls: add getpid syscall

src/tasking/syscalls.c

@@ -230,6 +230,11 @@ int sys_close(TrapFrame *tf)
     return -1;
 }
 
+int sys_getpid(TrapFrame *tf)
+{
+    return current->pid;
+}
+
 void handle_syscall(TrapFrame *tf)
 {
     /*
@@ -262,6 +267,10 @@ void handle_syscall(TrapFrame *tf)
         case 6://close
             tf->eax = sys_close(tf);
             break;
+        
+        case 20://getpid
+            tf->eax = sys_getpid(tf);
+            break;
         default:
             debug_log("unknown syscall: %X\n", tf->eax);
             tf->eax = -1;