Previously had 3 possibilities for handling exposed internal faces

  1. use default "oldInternalFaces"
  2. specify -patch, to use the specified (existing) patch
  3. specify -patches, to use the geometrically closest patches

Now relaxed the restriction on -patch to allow specification of a new
(not yet existing) patch name. This improves flexibility, but won't
catch typing mistakes.

Harmonize behaviour of -patches and -patch. When -patches is used to
specify a single, non-regex patch name, it now behaves identically to
-patch. Since the getList handling for options already allows special
treatment for single parameter lists, the following will work
identically:

      subsetMesh -patch  patch0
      subsetMesh -patches patch0
      subsetMesh -patches '( patch0 )'

In the future it might be reasonable to fully combine the behaviour of
'-patch' and '-patches' and treat them as aliases for each other.

ENH: support subsetMesh on a cellZone.

- when the '-zone' option is specified, the command argument is treated
  as the name (or names) of cellZones to be selected instead of as the
  name of the cellSet.

  The command argument can be a single word, regex, or list of
  word/regex.
  Eg,

      subsetMesh -zone -patch mypatch  mixer
      subsetMesh -zone -patch mypatch  '(mixer "moving.*" )'

STYLE: simplify set handling and other code cleanup in subsetMesh