po: Make it possible to pass custom options to msginit
* Makevars (MSGINIT_OPTIONS): New variable.
* Rules-quot: Use it when invoking msginit.
If one wants to use '--no-wrap' in MSGMERGE_OPTIONS and
XGETTEXT_OPTIONS, there should be a way to pass the option to
msginit. Reported by Ludovic Courtès in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-08/msg00004.html>.
