Revise a plan about link-set.

(I was ignorant 2 weeks ago.)
Use VAX_LABELOFFSET here; It needs to be the machine-specific one. Thanks
to gcc-4.8 for discovering the bug and to Atari for having a LABELOFFSET of
516 > 512 :-)
XXX: Perhaps we should put all those constants in <sys/bootblock.h> instead
of spreading them around.
add log_unknown_network
Add sysctl to selectively log arp packets from unknown network. (Adrien URBAN).
Stop building a.out kernels so that mkldscript.sh is no longer necessary.

Tested "build.sh release", and the default ELF GENERIC kernel loaded by
the native bootloader just works on LUNA-II.
Adjust flags to allow building with clang.
- Fix ntfs_ntlookupattr() to make the examples from the man page work.
- Fix ntfs_loadntnode() to always read full cluster to prevent buffer
  cache inconsistency.
- Change ntfs_vgetex() and ntfs_fget() to take a const attrname and
  always pass a possibly empty string.
- Change printf to dprintf.
- Get rid of dead fields i_next, i_prev of struct ntnode.
- Change ntfs_vgetex() to always load the ntnode and fnode.
- Take the vnode attributes from the NAME attribute.
- Remove now unused ntfs_times() and ntfs_filesize().
- Treat nodes without an unnamed DATA attribute as zero-sized regular files.
Change ntfs to vcache.
- Use (inumber, attrtype, attrname) as key.
- Inline ntfs_fget() and ntfs_frele() as they only get called once.
fix strict aliasing violations
move to the right offset