Fiddle with starting directory.

t/porting/podcheck.t, invoked without either '--add_link' or '--regen', is
documented to be callable either from the top-level directory:

  ./perl -Ilib t/porting/podcheck.t

... or from t/ via the harness (as in 'make test'):

  cd t;./perl harness porting/podcheck.t;cd -

I've begun to refactor code in podcheck.t into subroutines and to move those
subroutines into t/lib/PodcheckUtils.pm.  But those subroutines, as well as
code still left in podcheck.t, depend on functions require-ed in from
regen/regen_lib.pl.  This poses the problem of locating regen/regen_lib.pl:
Is it in ./ or in ../?  Depends on whether we have chdir-ed to t/ or not.

For the time being, invoking podcheck.t via the harness after having chdir-ed
to t/ is broken.  But the other invocation methods seem to be working.  So
let's continue along the path of encapsulating podcheck.t code into
subroutines and moving them into the module.  We'll come back to the harness
problem later.
Add PodcheckUtils.pm to MANIFEST.
Move some variable definitions closer to where they're used.

Identify variables needed within an 'if' block.
Fix global variables revealed by use of 'use strict' in PodcheckUtils.pm.
Encapsulate more code into check_all_files().

Substitute hashrefs for hashes for storing state:  a step on the road to an
object.
Move some variable declarations closer to where they're used.
More hash to hashref conversion.
Create package PodcheckUtils::Canonicalize;

Place canonicalize() into an object-oriented module.  This will enable us to
provide $vms_re and \%special_vms_files exactly once in the class's
constructor and simplify the invocation of $canon->canonicalize on individual
files.
Move suppressed() to PodcheckUtils.