LibC: Implement scandir(...) to enumerate directories.

I ran into a need for this when running  stress-ng against the system.
This change implements the full functionality of scandir, where it
accepts a selection callback, as well as a comparison callback.
These can be used to trim and sort the entries from the directory
that we are being asked to enumerate. A test was also included to
validate the new functionality.

Userland/Tests/LibC/TestLibCDirEnt.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <dirent.h>
+#include <string.h>
+
+TEST_CASE(scandir_basic_scenario)
+{
+    struct dirent** namelist = nullptr;
+    auto entries = scandir("/etc", &namelist, nullptr, nullptr);
+    EXPECT(entries > 0);
+    EXPECT_NE(namelist, nullptr);
+
+    bool found_passwd = false;
+    for (auto i = 0; i < entries; i++) {
+        if (strcmp(namelist[i]->d_name, "passwd") == 0) {
+            found_passwd = true;
+            break;
+        }
+        free(namelist[i]);
+    }
+    EXPECT(found_passwd);
+    free(namelist);
+}