Emacs: Add new wayland flavor

Emacs requires mutually exclusive build parameters depending on whether
the user is running under X11 or Wayland. Rather than require either
set of users to build a custom package, create a new wayland flavor, and
use default options appropriate for users running under X.

PR: 278167
Reviewed by: ashish, jbeich
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44917