emacs: Create an API for fetching parts as undecoded binary

emacs: Create an API for fetching parts as undecoded binary

The new function, `notmuch-get-bodypart-binary', replaces
`notmuch-get-bodypart-internal'.  Whereas the old function was really
meant for internal use in `notmuch-get-bodypart-content', it was used
in a few other places.  Since the difference between
`notmuch-get-bodypart-content' and `notmuch-get-bodypart-internal' was
unclear, these other uses were always confusing and potentially
inconsistent.  The new call clearly requests the part as undecoded
binary.

This is step 1 of 2 in separating `notmuch-get-bodypart-content' into
two APIs for retrieving either undecoded binary or decoded text.