Rename configurable directory variables and convert them to globals

Rename configurable directory variables and convert them to globals

Uploads arrive in Inbox_dir and are atomically transferred to Scratch_dir
for processing.  Uploaded files to be published are copied to Stage_dir
and atomically transferred to locations underneath Public_dir, while files
withdrawn from publication are atomically transferred to locations
underneath Archive_dir.  The new variable names better describe the
purposes of these directories.

These variables are also converted from file-scope lexicals to true global
variables, with access scoped lexically using Perl's "our" feature.