emacs: Fix two bugs in reply

emacs: Fix two bugs in reply

Bug 1: Replying from alternate addresses
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The reply code was inconsistent in its use of symbols and strings for
header names being passed to message.el functions. This caused the
From header to be lookup up incorrectly, causing an additional From
header to be added with the user's primary address instead of the
correct alternate address.

This is fixed by using symbols everywhere, i.e. never using strings
for header names when interacting with message.el.

This change also removes our use of `mail-header`, since we don't use
it anywhere else, and using assq makes it clear how the header lists
are expected to work.

Bug 2: Duplicate headers in emacs 23.2
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The message.el code in emacs 23.2 assumes that header names will
always be passed as symbols, so our use of strings caused
problems. The symptom was that on 23.2 (and presumably on earlier
versions) the reply message would end up with two of some headers.

Converting everything to symbols also fixes this issue.