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/VirtualFileSystem.h

@@ -78,6 +78,7 @@ private:
     friend class FileHandle;
 
     void enumerateDirectoryInode(InodeIdentifier, Function<bool(const FileSystem::DirectoryEntry&)>);
+    String absolutePath(InodeIdentifier);
     InodeIdentifier resolvePath(const String& path);
     InodeIdentifier resolveSymbolicLink(const String& basePath, InodeIdentifier symlinkInode);