Re-worked construction of .TH title line (closes #1210488).

Re-worked construction of .TH title line (closes #1210488).
Also, made comment generated at top of page include version info
(closes #1211254).

Here are the details about the refinements made to the
construction of the .TH title line:

  - "extra1" (which shows up in the center footer of each page):
    If a date cannot be found in the source, we now automatically
    generate a localized "long format" date

  - "extra2" (which shows up in the left footer):
    We now first search for "product version" info; then, if we
    can't find that, a "product name"; if we can't find that, we
    look for "other" info to use. And we can't find that, we leave
    it empty. The exact sequence of elements checked is this:

      1. productnumber in info or refentryinfo
      2. productnumber in info or referenceinfo of parent reference
      3. any refmeta/refmiscinfo that has class = 'version'
      4. productname in info or refentryinfo
      5. productname in info or referenceinfo of parent reference
      6. refmeta/refmiscinfo (first one)
      7. refnamediv/refclass (first one)

  - "extra3" (which shows up in the center header):
    The exact sequence of elements checked is now this:

      1. title in info or referenceinfo of parent reference
      2. refnamediv/refclass (first one)
      3. refmeta/refmiscinfo (first one)