Modify kill and exit functions on emacs notmuch mode

[Kiso Katsuyuki]

> ... but as someone(tm) may know, everyone have their own workflow
> ( https://xkcd.com/1172/ ) in their notmuch usage -- someone may
> e.g. start writing email based on arbitrary C code buffer, and
> after email send, return to that buffer instead of going to some
> other notmuch buffer.

I see. Many people may not feel unusefulness, as you said, and
I misunderstood that the selection priority of asterisk buffers like
"*notmuch-hello*" are lower than file buffers.
The problem seems to be my environmental problem because
there are no difference of priority between asterisk buffers and
file ones by default ('emacs -q').

> An alternative to this series would be adding a buffer selection
> hook variable which anyone can set to bre^H^H^H^improve their
> usage.

I appreciate your advice, but I am satisfied with this change and
will keep this change local now.

Warm Regards,
Kiso

-- 
-----------------------------------------------------------
木曽 勝之 （Katsuyuki Kiso）

mail: katsuyuki2388 at gmail.com
-----------------------------------------------------------