emacs: Build forwarded message buffer more directly

emacs: Build forwarded message buffer more directly

Previously, we used `message-forward' to build forwarded messages, but
this function is simply too high-level to be a good fit for some of
what we do.

First, since `message-forward' builds a full forward message buffer
given the message to forward, we have to duplicate much of the logic
in `notmuch-mua-mail' to patch the notmuch-y things into the built
buffer.

Second, `message-forward' constructs the From header from
user-full-name and user-mail-address.  As a result, if we prompt the
user for an identity, we have to parse it into name and address
components, just to have it put back together by `message-forward'.
This process is not entirely loss-less because
`mail-extract-address-components' does a lot of canonicalization
(since it's intended for displaying addresses, not for parsing them).

To fix these problems, don't use `message-forward' at all.
`message-forward' itself is basically just a call to `message-mail'
and `message-forward-make-body'.  Do this ourselves, but call
`notmuch-mua-mail' instead of `message-mail' so we can directly build
a notmuch-y message and control the From header.

This also fixes a bug that was a direct consequence of our use of
`mail-extract-address-components': if the user chose an identity that
had no name part (or the name part matched the mailbox), we would bind
user-full-name to nil, which would cause an exception in the bowels of
message-mode because user-full-name is expected to always be a string
(even if it's just "").