mailbox: Keep track of unread messages
mailbox: Keep track of unread messages

Sets the X-Unread header on new inbound messages. Also added
functions to set/read this header.

To be able to persist the header changes to disk, all messages loaded from
disk now gets a X-FilePath header that holds the path of the original file.

X-Unread, X-FilePath, X-P2POnly are removed from outbound messages.

LA5NTA/wl2k-go,LA5NTA/wl2k-go