LibC: Implement dirname() and basename()

And write section 3 man pages for them.

Base/usr/share/man/man3/basename.md

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+## Name
+
+basename - extract file name from a path
+
+## Synopsis
+
+```**c++
+#include <libgen.h>
+
+char* basename(char* path);
+```
+
+## Description
+
+Given a file path, `basename()` returns that file's name. `basename()` works
+purely lexically, meaning it only manipulates the path as a string, and does
+not check if such a file actually exists.
+
+A call to `basename()` may reuse and modify the passed in `path` buffer. Do not
+expect it to have the same value after calling `basename()`.
+
+## Return value
+
+`basename()` returns the file name as a string. This string may be allocated
+in static memory, or it may point to some part of the original `path` buffer.
+Do not `free()` the returned string, and do not `free()` the original `path`
+buffer while using the returned string.
+
+## Examples
+
+```c++
+#include <AK/LogStream.h>
+#include <libgen.h>
+
+int main()
+{
+    char path1[] = "/home/anon/ReadMe.md";
+    dbg() << basename(path1); // should be "ReadMe.md"
+
+    char path2[] = "foo/bar/";
+    dbg() << basename(path2); // should be "bar"
+
+    char path3[] = "foo";
+    dbg() << basename(path3); // should be "foo"
+
+    char path4[] = "/";
+    dbg() << basename(path4); // should be "/"
+}
+```
+
+## See also
+
+* [`dirname`(3)](dirname.md)

Base/usr/share/man/man3/dirname.md

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+## Name
+
+dirname - get a file's containing directory path
+
+## Synopsis
+
+```**c++
+#include <libgen.h>
+
+char* dirname(char* path);
+```
+
+## Description
+
+Given a file path, `dirname()` returns a path to the directory that contains the
+file. `dirname()` works purely lexically, meaning it only manipulates the path
+as a string, and does not check if such a file or its containing directory
+actually exist.
+
+A call to `dirname()` may reuse and modify the passed in `path` buffer. Do not
+expect it to have the same value after calling `dirname()`.
+
+## Return value
+
+`dirname()` returns the directory path as a string. This string may be allocated
+in static memory, or it may point to some part of the original `path` buffer.
+Do not `free()` the returned string, and do not `free()` the original `path`
+buffer while using the returned string.
+
+## Examples
+
+```c++
+#include <AK/LogStream.h>
+#include <libgen.h>
+
+int main()
+{
+    char path1[] = "/home/anon/ReadMe.md";
+    dbg() << dirname(path1); // should be "/home/anon"
+
+    char path2[] = "foo/bar/";
+    dbg() << dirname(path2); // should be "foo"
+
+    char path3[] = "foo";
+    dbg() << dirname(path3); // should be "."
+
+    char path4[] = "/";
+    dbg() << dirname(path4); // should be "/"
+}
+```
+
+## See also
+
+* [`basename`(3)](basename.md)