More work towards getting bash to build.

Implemented some syscalls: dup(), dup2(), getdtablesize().
FileHandle is now a retainable, since that's needed for dup()'ed fd's.
I didn't really test any of this beyond a basic smoke check.

LibC/dirent.cpp

@@ -3,6 +3,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <errno.h>
 #include <Kernel/Syscall.h>
 
 extern "C" {
@@ -20,6 +21,16 @@ DIR* opendir(const char* name)
     return dirp;
 }
 
+int closedir(DIR* dirp)
+{
+    if (!dirp || dirp->fd == -1)
+        return -EBADF;
+    int rc = close(dirp->fd);
+    if (rc == 0)
+        dirp->fd = -1;
+    return rc;
+}
+
 struct sys_dirent {
     ino_t ino;
     byte file_type;