Override draft headers with arguments. (closes #3580)

Override draft headers with arguments. (closes #3580)

Currently, if the -H option is used, it is not possible to redefine the
recipient addresses with positional argument. This patch fixes it as
well as CC, Bcc, and Subject headers.

This patch is based on the patch by Petr Písař at
http://dev.mutt.org/trac/attachment/ticket/3580/override_draft_headers_with_arguments.patch

This version of the patch merges the To, Cc, and Bcc commandline
arguments into the template, and fixes a few small problems with the
previous patch.

This patch also moves the fin close outside the "if (tempfile)" block,
since it is opened outside that block.