Use a different flag in mx_open_mailbox_append() when mailbox doesn't exist.

Use a different flag in mx_open_mailbox_append() when mailbox doesn't exist.

The previous commit re-used MUTT_NEWFOLDER, but the meaning of that
flag is slightly different: it causes mbox to use fopen with mode "w",
and is used only for the case of a brand-new mktemp-generated mbox.

Setting it for other non-existing mbox files leads to a race condition
between the stat and the fopen/lock, and so could end up truncating an
existing mailbox created in-between!

Create a different flag, MUTT_APPENDNEW to notify the open_append()
functions that the mailbox doesn't exist.  Change maildir and mh to
check for that flag to create their directory structures.