copy: add CH_VIRTUAL to allow to specify when use virtual headers

For example don't add "Tags:" header to the message when copy the
message to another folder. The virtual headers make sense only when
we display the message to user.

Signed-off-by: Karel Zak <kzak@redhat.com>
notmuch: update DB on save-command

Signed-off-by: Karel Zak <kzak@redhat.com>