LibC: Add function fdopendir

This adds the function fdopendir and refactors opendir so that opendir
just opens a file descriptor and passes the file descriptor onto
fdopendir.

Userland/Libraries/LibC/dirent.cpp

@@ -24,6 +24,13 @@ extern "C" {
 DIR* opendir(const char* name)
 {
     int fd = open(name, O_RDONLY | O_DIRECTORY);
+    if (fd == -1)
+        return nullptr;
+    return fdopendir(fd);
+}
+
+DIR* fdopendir(int fd)
+{
     if (fd == -1)
         return nullptr;
     DIR* dirp = (DIR*)malloc(sizeof(DIR));