renamimg mime_node_* to notmuch_mime_node_*

[David Bremner]

I've started working on allowing gzipped message files. The access in
libnotmuch is relatively centralized in message-file.c, and I've managed
to convert that to allow gzipped files [1]. However this is still direct
file access in the notmuch CLI, and at least mime-node.c and
notmuch-show.c will need to be updated. notmuch-show looks like a simple
gzopen / gzreadline substitution can do the trick, but mime-node.c will
require the same slightly trick work as I already did in
lib/message-file.c (_message_file_open_stream). I'd like to avoid
duplicate that. There seems to be two approaches: export some of the
_notmuch_message_file API and re-use that in mime-node.c, or put the
mime_node_* functions in the library. I'm not sure yet which of these
options is cleaner; I'd prefer not to expose GMime types as part of the
libnotmuch API.  Does anyone have any arguments either way? Perhaps some
other motivation for pushing the mime-node stuff into libnotmuch?

CC'ing dkg as the last person to meaningfully edit mime-node.c

[1]: the (very) WIP is available at https://salsa.debian.org/bremner/notmuch/tree/wip/gzip