Update and merge mail/mu and mail/mu4e

Hi,

I send you an email to ask a review for an update to mail/mu at 
https://reviews.freebsd.org/D46961

It is my first time to deal with port.

Let me introduce few context about this review.

- mail/mu is a tool to manage maildir. Its frontend is mail/mu4e 
  for emacs. Actually freebsd ports has both ports.

- I contact the ports maintainer, by mail, by reporting a bug at 
  https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280678 and a 
  previous review at   https://reviews.freebsd.org/D45539 without 
  any  response.

- My review consist in an urgent up-to-date and merging mail/mu 
  and mail/mu4e, the latter becomes a build option (activated by 
  default) because the developpment of both packages are 
  intimately  construct as well as in the default building system 
  (meson).

- Note, I was unable to use USES += emacs which yields an a ninja 
  error thus I pu in in  BUILD_DEPENDS.

My questions are:

- What do you think of merging both package ?

- Can you review the code ?

Any comments is welcome!

Best,
RwN.