Implement symlink() that always creates file symlinks. Fails with ENOSYS
if symlinks are disabled or unsupported.
Note: CreateSymbolicLinkW() was introduced with symlink support in Windows
Vista. For compatibility with Windows XP, we need to load it dynamically
and fail gracefully if it isnt's available.
Signed-off-by: Karsten Blees <blees@dcon.de>