Flavorize and modernize Emacs ports

This is an addendum to r460621 to fix the histories of Mk/Uses/emacs.mk
(copied from Mk/bsd.emacs.mk) and japanese/migemo-emacs (copied from
japanese/migemo-emacs23).

• Convert USE_EMACS to USES=emacs
• Rename japanese/migemo-emacs23 to japanese/migemo-emacs

Differential Revision: https://reviews.freebsd.org/D13506