Fix write of Symlink files

[antoineco]

Fixes #15

writeSVR4Header() currently omits the following for files with mode Symlink:

• The file's Size must match the link target (Linkname)
• The link target must be written after the pad that follows the file's name

Both match the way readSVR4Header() translates the final read into a Linkname (and returned file size of 0).