emacs: Simplify and fix `notmuch-mua-prompt-for-sender'

`notmuch-mua-prompt-for-sender' is over-engineered and often wrong.
It attempts to detect when all identities have the same name and
specialize the prompt to just the email address part.  However, to do
this it uses `mail-extract-address-components', which is meant for
displaying email addresses, not general-purpose parsing, and hence
performs many canonicalizations that can interfere with this use.  For
example, configuring notmuch-identities to ("Austin
<austin@example.com>"), will cause `notmuch-mua-prompt-for-sender' to
lose the name part entirely and return " <austin@example.com>".

This patch rewrites `notmuch-mua-prompt-for-sender' to simply prompt
for a full identity when notmuch-identities is configured, or to
prompt for a sender address when it isn't.

The original code also did several strange things, like using `eval'
and specifying that this function was interactive.  As a side-effect,
this patch fixes these problems.  And it adds a docstring.