Add a "pwd" utility to userland.

It's implemented as a separate process. How cute is that.
Tasks now have a current working directory. Spawned tasks inherit their
parent task's working directory.
Currently everyone just uses "/" as there's no way to chdir().

VirtualFileSystem/FileHandle.h

@@ -16,6 +16,8 @@ public:
 
     ByteBuffer readEntireFile();
 
+    String absolutePath() const;
+
 #ifdef SERENITY
     int fd() const { return m_fd; }
     void setFD(int fd) { m_fd = fd; }