Support for changes to OS X Mavericks VNOP_SYMLINK() function.

Add support for an extra argument to afs_symlink() to return the
newly-created symlink vnode if requested (this is needed on OS X
Mavericks).  On OS X Mavericks return the newly-created symlink vnode in
the symlink vnops functions, on all other platforms ignore it.

It turns out that technically OS X has required the symlink to be
created for a while, but code inside of symlink() would call namei() on
the symlink name if the returned vnode point was NULL.  The difference
is that on Mavericks the Manditory Access Control Framework has been
enabled, and that turns on some extra code which unconditionally calls
vnode_mount() on the returned vnode pointer, which ends up causing a
panic

Reviewed-on: http://gerrit.openafs.org/10474
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 3f4c1099b7b2d1467b1f5b701ea2f953fec20dc0)

Change-Id: Ib8bac6fd4ed8fe5c2e9567431f7d03fdab50cd50
Reviewed-on: http://gerrit.openafs.org/10519
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>