smbd: Simplify dropbox special case in unix_convert

smbd: Simplify dropbox special case in unix_convert

EACCESS needs special treatment: If we want to create a fresh file,
return OBJECT_PATH_NOT_FOUND, so that the client will continue creating
the file. If the client wants us to open a potentially existing file,
we need to correctly return ACCESS_DENIED.

This patch makes this behaviour hopefully a bit clearer than the code
before did.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Aug 26 12:14:26 CEST 2013 on sn-devel-104

The last 2 patches address bug #10114 - Dropbox (write-only-directory) case
isn't handled correctly in pathname lookup.