cleanup: convert all instances of "hidden" to "lower".

cleanup: convert all instances of "hidden" to "lower".

The unionfs code contained many references to symbols whose name had the
string "hidden" in it, referring to, for example, a "hidden inode" below the
unionfs inode.  The term "hidden" was there for historical reasons and was a
misnomer, implying that the objects at unionfs's lower branches were somehow
hidden or unavailable.  This was not just incorrect, but confusing.  The
lower objects are just that: lower.  They are not hidden from users.  In
fact, users can 'cd' to lower branches and manipulate files directly there.
This long patch renames all such mis-named symbols, and re-indents the code
as needed to comply with coding standards.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>