LDP: Support po4a 0.62

LDP: Support po4a 0.62

po4a 0.62 is available in Homebrew on macOS.
I noticed some points which does not work with po4a 0.62.

I see the following problems at least:

* '\c' syntax is not handled properly especially the next line
  starts with roff directive (e.g., .RI).
  I haven't found a solution thru po4a, so workaround.patch replaces
  such cases with equivalent syntax that po4a can treat properly.
* Addendum is not processed when "add_$lang" is placed after "opt:"
  in po4a configuration files. I confirmed that addendum is not
  processed using po4a --debug option. We can avoid this by placing
  "opt:" after "add_$lang" part.

I think they are bugs of po4a 0.62 but it would be nice if this
workaround works.