exists is bogus. The goal here is to produce a synthetic link count
which won't confuse fts and similar routines which "know" that
directories with a link count of 2 don't have subdirectories (and
thus, they can avoid having to stat every entry in the directory
looking for subdirectories which aren't there).
We know that non-UNIX filesystem implementations may return a link
count of `1' for directories with an indeterminate number of
subdirectories; if either the upper or lower layer returns a link
count of `1', return a link count of 1. If both layers return a link
count of 2, return a link count of 2; otherwise, return the sum of the
link count of both layers.
Also, fix PR7430: unionfs ignores read-only mounts. Check for
MNT_RDONLY in union_lookup (more-or-less as in layer_lookup) as well
as union_access() and union_setattr().
Note that a read-only union layer may still cause side effects on the
underlying filesystems... Most notably, we'll still attempt to create
shadow directories in the upper layer. Also, of course, we'll
side-effect atimes in the lower layer.
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