imap-send: create target mailbox if it is missing

Some MUAs delete their "drafts" folder when it is empty, so
git imap-send should be able to create it if necessary.

This change checks that the folder exists immediately after
login and tries to create it if it is missing.

There was some vestigial code to handle a [TRYCREATE] response
from the server when an APPEND target is missing. However this
code never ran (the create and trycreate flags were never set)
and when I tried to make it run I found that the code had already
thrown away the contents of the message it was trying to append.

Signed-off-by: Tony Finch <dot@dotat.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>