Move vnode macros to their own directories

Move vnode macros to their own directories

The tree is inconsistent whether macros for access to vnodes are
provided by the OS directories, or in afs_osi.h. This makes things
very confusing, especially in the Linux case where macros are
provided in afs_osi.h, and then promptly redefined in
LINUX/osi_machdep.h

Adopt a convention where default macros are conditionally provided
by osi_machdep.h. Where these aren't wanted, they should be disabled
in osi_machdep.h, and OS specific versions provided in the individual
OS's directory.

Change-Id: I47bc969af3f6670db9348689b5ce60181fd44f21
Reviewed-on: http://gerrit.openafs.org/784
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>