This patch adds a mx_open_new_message() flag, M_SET_DRAFT.  It also adds
a MESSAGE->flags.draft flag.

mutt_write_fcc() passes the M_SET_DRAFT flag to mx_open_new_message(),
which then sets MESSAGE->flags.draft.  Then, imap_append_message() is
able to see this flag and so adds the \Draft flag.

The imap_append_message() function started to have a bit too many flags,
so this version of the patch separates out the flag generating code into
a simpler version.