header buffer
When we're doing a search while the results of a previous search are still
coming in from the backend, this way we can tell the frontend that this is
the right time to clear the buffer (previously, sometimes the results would
be mixed).
- org-mu4e.el: add "mu4e:"-link type and handlers to org-mode
- Makefile.am: add org-mu4e.el to package
- mu4e-hdrs.el: register handler functions for server data *outside* header
mode, so they can be used independently (and we can link to messages even
when mu4e is not active already)
- mu4e-view.el, mu-server.1, mu-proc.el, mu-cmd-server.c: support viewing
messages by msgid (rather than only by docid); document it
