[MERGE] Improve Porting/sync-with-cpan

The original goal of this sequence of patches was to address RT#130375. That
ticket proposes extending the release manager's guide with discussion of
what regen/lib_cleanup.pl does, and why it might sometimes be needed, and
thus why Porting/sync-with-cpan might yield a porting-test failure when
t/porting/regen.t detects that it needs to be run.

However, I find the RMG is already relatively bulky, and therefore hard to
navigate; so adding more text to it would exacerbate that problem. In
addition, it seems much better to merely fix this problem: we can always run
regen/lib_cleanup.pl, and if it makes any changes, those changes can be
committed along with the updated CPAN module being imported.

This merge also contains a few other improvements to Porting/sync-with-cpan:

- The log file it generates is now listed in .gitignore

- It allows specifying the module name with "-" rather than "::" as a name
  separator

- A bug that prevented the new module's tests from being run is now fixed

- The MANIFEST is now rewritten to accommodate any file deletions introduced
  by the updated CPAN module

- The messages emitted have been reworked slightly for clarity; in
  particular, "Fixing MANIFEST" will no longer be followed by a confusing
  and scary "'MANIFEST' is NOT sorted properly" message

- It now takes a --jobs option, which if supplied is used both for executing
  make and running the module's tests

- The --tarball option now works when the tarball isn't in the current
  directory

- It runs a few additional preflight checks that detect mistakes I found
  myself making

These changes have been tested by using Porting/sync-with-cpan to import
Test::Simple into blead as of 54f6f377a29af7d60918c003fc2c462439c340bd,
since that was the triggering case for RT#130375.