share/mk/doc.translate.mk: Improve make po

• Create .pot (Portable Object Template) files for the en_US documents Do not ignore en_US language and generate a .pot file instead of a .po. These are used as template for tools like Zanata.

• Execute msgattrib even on a new [.po|.pot] file This will be useful to wrap long message lines and avoid bigger diffs in the future.

Approved by: wosch
Differential Revision: https://reviews.freebsd.org/D16726